Tape drive and printing apparatus

ABSTRACT

A tape drive for use in for example transfer printing apparatus to drive a printer ribbon. The printer ribbon is mounted on two spools each of which is driven by a respective stepper motor. A controller controls the energisation of the motor such that the ribbon is transported in at least one direction between spools mounted on the spool support. The controller is operative to energise both motors to drive the spools of ribbon in the direction of ribbon trasport to achieve push-pull operations. Ribbon tension is monitored to enable accurate control of ribbon supply and ribbon takte up, the ribbon tension being monitored by monitoring power supply to the two stepper motors.

[0001] This invention relates to tape drive and printing apparatus andoperation methods and in particular to such apparatus and methods whichmay be used in transfer printers, that is printers which make use ofcarrier-supported inks.

[0002] In transfer printers, a tape which is normally referred to as aprinter ribbon and carries ink on one side is presented within a printersuch that a printhead can contact the other side of the ribbon to causethe ink to be transferred from the ribbon on to a target substrate offor example paper or a flexible film. Such printers are used in manyapplications. Industrial printing applications include thermal transferlabel printers and thermal transfer coders which print directly on to asubstrate such as packaging materials manufactured from flexible film orcard.

[0003] Ink ribbon is normally delivered to the end user in the form of aroll wound onto a core. The end user pushes the core on to a tape spool,pulls a free end of the roll to release a length of ribbon, and thenengages the end of the tape with a further spool. Generally the spoolsare mounted on a cassette which can be readily mounted on a printingmachine. The printing machine includes a transport means for driving thetwo spools, so as to unwind ribbon from one spool and to take up ribbonon the other spool. The printing apparatus transports ribbon between thetwo spools along a predetermined path past the printing head.

[0004] Known printers of the above type rely upon a wide range ofdifferent approaches to the problem of how to drive the ribbon spools.Some rely upon stepper motors,:others on DC motors to directly orindirectly drive the spools. Generally the known arrangements drive onlythe spool on to which ribbon is taken up (the take-up spool) and relyupon some form of “slipping clutch” arrangement on the spool from whichribbon is drawn (the supply spool) to provide a resistive force so as toensure that the ribbon is maintained in tension during the printing andribbon winding processes and to prevent ribbon over run when the ribbonis brought to rest. It will be appreciated that maintaining adequatetension is an essential requirement for the proper functioning of theprinter.

[0005] As a roll of ribbon is gradually used by the printer, the initialoutside diameter of the supply spool decreases and the initial outerdiameter of the take-up spool increases. In slipping clutch arrangementswhich offer an essentially constant resistive torque, the ribbon tensionwill vary in proportion to the diameter of the spools. Given that it isdesirable to use large supply spools so as to minimise the number oftimes that a ribbon roll has to be replenished, this is a seriousproblem particularly in high speed machines where rapid ribbon transportis essential.

[0006] Dynamically changing ribbon tension gives rise to demands fortight tolerances for the torque delivered by the slipping clutch. Suchtolerances are difficult to maintain as wear in the slipping clutch overtime tends to change the resistive force exerted by the clutch. If theclutch force is too great the ribbon transport system may haveinadequate power to drive the ribbon throughout the range of spooldiameters from a new supply roll to an empty supply roll. Too littleclutch force and slack in the ribbon could result in over run of thesupply spool. Given these constraints, typical printer designs havecompromised performance by way of limiting the rate of acceleration, therate of deceleration, and the maximum speed capability of the ribbontransport system. Overall printer performance has as a result beencompromised.

[0007] Representative examples of conventional printing apparatus aredescribed in U.S. Pat. No. 4,000,804, U.S. Pat. No. 4,294,552, U.S. Pat.No. 4,479,081, U.S. Pat. No. 4,788,558 and British patent 2310405.

[0008] The system of U.S. Pat. No. 4,000,804 describes an arrangementfor transferring a ribbon from a supply spool to a take-up spool whichincludes a pair of electric motors each one of which is connected to acorresponding spool shaft. The motors are direct current (DC) motors.The motor connected to the take-up spool is supplied by a constantcurrent generator so as to wind up the ribbon with a substantiallyconstant torque. The motor connected to the supply spool is supplied bya constant voltage generator so as to keep the ribbon tensioned duringribbon transfer. A change-over device alternates the function of the twospools when the ribbon is fully wound on the take-up spool. With thedescribed arrangement, no account is taken of the change in diameters ofthe supply and take-up spools during ribbon transfer and thus ribbontension varies substantially during the course of the full transfer ofthe ribbon from the supply spool to the take-up spool.

[0009] U.S. Pat. No. 4,294,552 discloses a bi-directional ribbon drivein which two spools are driven by respective stepper motors. The take-upspool is driven by its stepper motor, but the supply spool motor is feda low level “drag” current to maintain the ribbon in tension. The motorsare not actively controlled to compensate for spool diameter variations.

[0010] U.S. Pat. No. 4,479,081 describes an arrangement in which twostepper motors are provided, one driving the take-up spool and the othercoupled to the supply spool. Feed back signals provide an indication ofthe angular velocity of the supply spool and a function table providesinformation on the rate of stepping pulses to be applied to the take-upspool. The ribbon is driven by the stepper motor driving the take-upspool, the other motor acting as a feed back transducer to enableappropriate control of the motor driving the take-up spool to takeaccount of changing spool diameters whilst maintaining a constant ribbonspeed. Thus although this arrangement does avoid the need for example ofa capstan drive interposed between the two spools so as to achievereliable ribbon delivery speeds, only one of the motors is driven todeliver torque to assist ribbon transport. There is no suggestion thatthe apparatus can operate in push-pull mode, that is the motor drivingthe take-up spool operating to pull the ribbon and the motor driving thesupply spool operating to push the associated spool in a direction whichassists tape transport.

[0011] U.S. Pat. No. 4,788,558 describes a ribbon drive mechanism inwhich two DC motors are provided, one driving the take-up spool and onedriving the supply spool. Ribbon is delivered by a further drive rollerdriven by a stepper motor. The supply spool DC motor acts as a brake anddoes not assist in tape transport. Thus this is a conventionalarrangement in which a capstan roller is used to control ribbon deliveryspeed. With such an arrangement it is a relatively simple matter asdescribed to provide feedback information concerning the magnitude ofthe ribbon spools so as to maintain a desired ribbon tension, but theoverall system is complex.

[0012] GB 2310405 describes a bi-directional printer ribbon drivemechanism in which a stepper motor drives a take-up spool. Accuratecontrol of ribbon delivery is achieved by providing an idler rollerwhich rotates in contact with the ribbon and thus enables a directmeasurement of ribbon transport speed. The provision of such an idlerroller and associated components adds to overall system complexities andcost.

[0013] None of the known arrangements is capable of coping well with therequirements of high speed industrial transfer printing systems. Suchsystems generally operate in one of two manners, that is eithercontinuous printing or intermittent printing. In both modes ofoperation, the apparatus performs a regularly repeated series ofprinting cycles, each cycle including a printing phase during which inkis being transferred to a substrate, and a further non-printing phaseduring which the apparatus is prepared for the printing phase of thenext cycle.

[0014] In continuous printing, during the printing phase a stationaryprinthead is brought into contact with a printer ribbon the other sideof which is in contact with a substrate on to which an image is to beprinted. (The term “stationary” is used in the context of continuousprinting to indicate that although the printhead will be moved into andout of contact with the ribbon, it will not move relative to the ribbonpath in the direction in which ribbon is advanced along that path). Boththe substrate and printer ribbon are transported past the printhead,generally but not necessarily at the same speed. Generally onlyrelatively small lengths of the substrate which is transported past theprinter head are to be printed upon and therefore to avoid gross wastageof ribbon it is necessary to reverse the direction of travel of theribbon between printing operations. Thus in a typical printing processin which the substrate is travelling at a constant velocity, theprinthead is extended into contact with the ribbon only when theprinthead is adjacent regions of the substrate to be printed.Immediately before extension of the printhead, the ribbon must beaccelerated up to for example the speed of travel of the substrate. Theribbon speed must then be maintained at the constant speed of thesubstrate during the printing phase and, after the printing phase hasbeen completed, the ribbon must be decelerated and then driven in thereverse direction so that the used region of the ribbon is on theupstream side of the printhead. As the next region of the substrate tobe printed approaches, the ribbon must then be accelerated back up tothe normal printing speed and the ribbon must be positioned so that anunused portion of the ribbon close to the previously used region of theribbon is located between the printhead and the substrate when theprinthead is advanced to the printing position. Thus very rapidacceleration and deceleration of the ribbon in both directions isrequired, and the ribbon drive system must be capable of accuratelylocating the ribbon so as to avoid a printing operation being conductedwhen a previously used portion of the ribbon is interposed between theprinthead and the substrate.

[0015] In intermittent printing, a substrate is advanced past aprinthead in a stepwise manner such that during the printing phase ofeach cycle the substrate and generally but not necessarily the ribbonare stationary. Relative movement between the substrate, ribbon andprinthead are achieved by displacing the printhead relative to thesubstrate and ribbon. Between the printing phase of successive cycles,the substrate is advanced so as to present the next region to be printedbeneath the printhead and the ribbon is advanced so that an unusedsection of ribbon is located between the printhead and the substrate.Once again rapid and accurate transport of the ribbon is necessary toensure that unused ribbon is always located between the substrate andprinthead at a time that the printhead is advanced to conduct a printingoperation.

[0016] The requirements in terms of ribbon acceleration, deceleration,speed and positional accuracy of high speed transfer printers is suchthat the known drive mechanisms have difficulty delivering acceptableperformance with a high degree of reliability. Similar constraints alsoapply in applications other than high speed printers. Accordingly it isan object of the present invention to provide a tape drive which can beused to deliver printer ribbon in a manner which is capable of meetingthe requirements of high speed production lines, although the tape driveof the present invention may of course be used in other applicationswhere similar high performance requirements are demanded.

[0017] According to the present invention, there is provided a tapedrive comprising two motors at least one of which is a stepper motor,two tape spool supports on which spools of tape may be mounted, eachspool support being drivable by a respective motor, and a controller forcontrolling the energisation of the motors such that tape may betransported in at least one direction between spools mounted on thespool supports, wherein the controller is operative to energise bothmotors to drive the spools of tape in the direction of tape transport.

[0018] A tape drive in accordance with the present invention relies uponboth the motors which drive the two tape spools to drive the tape duringtape transport. Thus the two motors operate in push-pull mode. Thismakes it possible to achieve very high rates of acceleration anddeceleration. Tension in the tape being transported is determined bycontrol of the drive motors and therefore is not dependent upon anycomponents which have to contact the tape between the take-up and supplyspools. Thus a very simple overall mechanical assembly can be achieved.Given that both motors contribute to tape transport, relatively smalland therefore inexpensive and compact motors can be used.

[0019] The actual rotational direction of each spool will depend on thesense in which the tape is wound on each spool. If both spools are woundin the same sense then both spools will rotate in the same rotationaldirection to transport the tape. If the spools are wound in the oppositesense to one another, then the spools will rotate in opposite rotationaldirections to transport the tape. In any configuration, both spoolsrotate in the direction of tape transport.

[0020] Preferably the controller is arranged to control the motors totransport tape in both directions between the spools. The motors mayboth be stepper motors and the controller may be operative to monitortension in a tape being transported between spool mounted on the spoolsupport and to control the motors to maintain the monitored tensionbetween predetermined limits. Means are preferably provided to monitorthe power supply to at least one of the motors and to calculate anestimate of tape tension from the monitored power. For example, wheretwo stepper motors are provided, a power supply may deliver power to astepper motor drive means which supplies current sequentially towindings of the stepper motors, the power being monitored by means formonitoring the magnitude of voltage and/or current supplied to themotors and/or the motor drive means. It will be appreciated thatdependent upon the load applied to the motors the current and voltagedelivered to the motor windings will both vary, irrespective of the typeand nature of the motor drive means used. For this reason it ispreferred to provide a regulated power supply which supplies asubstantially constant voltage to the stepper motor drive means and tomonitor the magnitude of current supplied to the stepper motor drivemeans from the power supply.

[0021] Preferably each stepper motor is energised by a respective motordrive circuit, a respective low resistance resistor being connected inseries with each motor drive circuit, and voltage signals developedacross the series resistors being monitored to monitor the currentsupplied to the motors. The voltage signals may be converted to digitalsignals for supply to a microcontroller which controls the generation ofmotor control pulse trains-which are applied to the motor drivecircuits. The current may be monitored over a predetermined period oftime and preferably is monitored only during periods in which tapetransport speed is substantially constant. The predetermined period oftime may correspond to a predetermined length of tape transport.

[0022] Calibration data may be recorded for the or each stepper motor,the calibration data representing power consumption for the steppermotor at each of a series of step rates under no tape load conditions,and a measure of tape tension may be calculated by reference to ameasure of motor step rate, the calibration data related to the steprate, and the power consumed by the motor.

[0023] The outside diameters of the tape spool may be directly monitoredand the tape tension calculated to take into account the monitoreddiameters. The outside diameters may be monitored for each of aplurality of diameters which are mutually inclined to each other so asto enable the detection of spool eccentricity and therefore enable anaccurate calculation of the spool circumference.

[0024] A measure of tension t may be calculated from measures of powersupplied to the two motors, measures of the spool radii, calibrationfactors for the two motors related to the step rate of the motors. Acalibration scaling factor may also be used to translate the calculatedtension into a more interpretable value. Preferably the controllerimplements a control algorithm to calculate a length of tape to be addedto or subtracted from the tape extending between the spools in order tomaintain the value t between predetermined limits and to control thestepper motors to add or subtract the calculated length of tape to thetape extending between the spools. Alternatively, a measure of thedifference between the current supplied to the two motors may be derivedand stepping of the motors may be controlled dependent upon thedifference measure. It will be appreciated that the difference measurecould simply be the result of subtracting one current from the other orcould relate to the ratio of the two measured currents. Motor speed maybe maintained constant during a period in which the difference measureis within each of a series of tolerance bands defined between upper andlower limits, and the tolerance bands may be adjusted in dependence uponthe ratio of the outside diameters of the spools. The controlling meansmay implement a control algorithm to calculate a length of tape to beadded to or subtracted from the tape extending between the spools inorder to maintain the difference measure between the upper and lowerlimit and to control the stepper motors to add or subtract thecalculated length of tape to the tape extending between the spools.

[0025] A value corresponding to tape width may be input and thepredetermined limit adjusted to take account of that tape width. Forexample, the control algorithm may comprise gain constants, and the gainconstants may be adjusted to take account of tape width. The controlalgorithm may operate cyclically such that during one cycle the lengthof tape to be added or subtracted is calculated and during a subsequentcycle the motors are controlled to adjust the amount of tape between thespools. This approach is adopted because, as it will be appreciated thatalthough the length of tape between the spools is to a firstapproximation independent of tension, stretching of the tape will meanthat if tape is added to the length of tape extending between the spoolsthis will be taken up by a reduction in stretch until the tensionbecomes zero. It will be further appreciated that for a given tension, anarrower tape will stretch more than a wider tape, therefore a change intension, caused by the addition or subtraction of tape between thespools, will be less for a narrower tape than for a wider tape.

[0026] Tension monitoring males it possible to generate afault-indicating output if the measure of tension falls below a minimumacceptable limit to indicate for example a tape breakage.

[0027] Spool diameters may be monitored using an optical sensing systemincluding at least one light emitter and at least one light detectorarranged such that an optical path is established therebetween, atransport mechanism supporting at least one part of the optical sensingsystem and drivable so as to cause the optical path to sweep across aspace within which spools to be measured will be located, and acontroller operative to control the transport mechanism, to detectpositions of the transport mechanism in which the output of the detectorchanges to indicate a transition between two conditions in one of whichthe optical path is obstructed by a spool and in the other of which theoptical path is not obstructed by that spool, and to calculate the spooldiameters from the detected positions of the transport mechanism inwhich the detector output changes.

[0028] One of the emitter and detector may be mounted on the transportmechanism, the other being fixed in position relative to the spools oftape, or alternatively both the emitter and detector may be mounted onthe transport mechanism, the optical path between the emitter anddetector being established by a mirror located at the side of the spoolsremote from the transport mechanism and arranged to reflect light fromthe emitter back to the detector. Spool diameters may be measured withthe spools in a first position, the spools may then be rotated so thatone spool rotates by for example 30°, the diameters may be measuredagain, and so on. This makes it possible to accurately assess spooleccentricity and outer circumference.

[0029] The present invention has particular applicability where thetransport mechanism is a printhead transport mechanism of a transferribbon printer. The ratio of spools in such a machine can be calculatedon the basis of the output of the diameter measuring means. The ratiocalculating means may comprise means enabling a first stepper motordriving a take up spool and disabling a second stepper motor driving asupply spool such that the second stepper motor acts as a generator,means for generating pulses from the second stepper motor, the pulserate being proportional to motor speed, means for detecting thegenerated pulses to produce a measure of the rotation of the secondstepper motor, means for monitoring stepping of the first stepper motorto produce a measure of the rotation of the first stepper motor, andmeans for comparing the measures of the rotations of the motors tocalculate the ratio of the outside diameters of the spools.

[0030] After a number of operating cycles of the tape drive, in whichtape is transported between the spools, an updated diameter for at leastone of the spools may be calculated from a ratio between the spooldiameters as initially monitored, a current ratio between the spooldiameters, and the diameter of at least one spool as initiallymonitored.

[0031] Where the tape drive in accordance with the invention isincorporated in a transfer printer for transferring ink from a printerribbon to a substrate which is transported along a predetermined pathadjacent to the printer, the tape drive acting as a printer ribbon drivemechanism for transporting ribbon between first and second ribbonspools, the printer may further comprise a print head arranged tocontact one side of the ribbon to press an opposite side of the ribboninto contact with a substrate on the predetermined path, a printheaddrive mechanism for transporting the printhead along a track extendinggenerally parallel to the predetermined substrate transport path and fordisplacing the printhead into and out of contact with the ribbon, and acontroller controlling the printer ribbon and printhead drivemechanisms, the controller being selectively programmable either tocause the ribbon to be transported relative to the predeterminedsubstrate transport path with the printhead stationery and displacedinto contact with the ribbon during printing, or to cause the printheadto be transported relative to the ribbon and the predetermined substratetransport path and to be displaced into contact with the ribbon duringprinting.

[0032] The drive mechanism may be bidirectional such that ribbon may betransported from the first spool to the second spool and from the secondspool to the first.

[0033] Where the printhead is mounted on a printhead carriage that isdisplaceable along the track, first and second carriages may be providedwhich are interchangeable and shaped such that with one carriage inposition on the track the printhead is disposed so as to enable printingon a substrate travelling in one direction along the substrate transportpath and with the other carriage in position on the track the printheadis disposed so as to enable printing on a substrate travelling in theother direction along the substrate path.

[0034] The tape drive may be incorporated in a printing apparatuscomprising a housing, a printhead mounted on a printhead supportassembly which is displaceable relative to the housing in a directionparallel to a print ribbon path along which a ribbon is driven by thetape drive, a first drive mechanism for displacing the printhead supportrelative to the housing, a roller which in use supports a substrate tobe printed on the side of the ribbon path remote from the print head, asecond drive mechanism for displacing the printhead relative to theprinthead support assembly to a printing position in which a portion ofthe printhead bears against the roller or any substrate or ribboninterposed between the printhead and roller, and a controller foradjusting the first drive mechanism to adjust the angular position ofthe printhead relative to the rotation axis of the roller.

[0035] Preferably the printhead is mounted on a printhead supportassembly which is displaceable relative to the housing in a directionparallel to a print ribbon path along which a ribbon is driven by thetape drive, a first drive mechanism for displacing the printhead supportrelative to the housing, a peel off roller mounted on the printheadsupport assembly and displaceable with the printhead in the saidparallel direction, and a second drive mechanism for displacing theprinthead relative to the printhead support assembly and peel off rollerbetween a ready to print position adjacent the print ribbon path and aprinting position in which the printhead would contact a print ribbon onthe path, wherein a cam mechanism is provided which is engaged as aresult of displacement of the printhead support assembly to apredetermined position and when engaged causes retraction of theprinthead away from the ready to print position to a position spacedfrom the peel roller and the print ribbon path.

[0036] The cam mechanism may comprise a plate mounted in the housing anddefining a slot, and a pin extending from a pivotal member mounted onthe printhead support assembly, engagement of the pin in the slot as aresult of displacement of the printhead support assembly to thepredetermined position causing the pivotal member to rotate from a firstposition in which it supports the printhead to a second position inwhich the printhead is free to move to the position spaced from the peelroller and the print ribbon path.

[0037] The pivotal member may be mounted on a displaceable membermounted on the printhead support assembly, displacement of thedisplaceable member from a retracted to an extended position when thepivotal member is in the first position causing the printhead to movefrom the ready to print from the printing position.

[0038] The printing apparatus may further comprise means for applyingthe printhead to a ribbon supported in the drive mechanism, theprinthead comprising an array of printing elements each of which may beselectively energised to release ink from a portion of ribbon contactedby that element, and a controller for controlling energisation of theprinting elements and the advance of the ribbon so as to perform aseries of printing cycles each of which includes a printing phase duringwhich relative movement between the printhead and ribbon results in theprinthead traversing a predetermined length of ribbon and a non-printingphase during which the ribbon is advanced a predetermined distancerelative to the printhead, wherein the controller is arrangedselectively to energise different groups of printing elements duringsuccessive printing cycles, the groups of elements being distributed onthe printhead such that different groups contact different portions ofthe ribbon, and the controller is arranged to advance the ribbon suchthat the said predetermined distance of ribbon advance is less that thesaid predetermined length of ribbon, the groups of printing elementsbeing energised such that that ribbon is advanced by at least saidpredetermined length of ribbon in the interval between any two printingphases in which the same group of printing elements are energised. Twogroups of printing elements may be provided such that the distance ofribbon advance may be as little half the predetermined length of ribbon.

[0039] Given the fundamental differences between continuous andintermittent printing systems as described above, it has been industrypractice to provide printing apparatus which is capable either of use ina continuous printing application or for use in an intermittent printingapplication but not to provide a printer with the versatility to performboth functions. The fundamental difference between the two types ofprinting apparatus required for these two applications is that in one(continuous printing) the printhead is stationary (using that term inthe manner discussed above) whereas in the other (intermittent) theprinting head must be displaceable. As a result, when a particularproduction line is converted from for example an intermittent printingapplication to a continues printing application it is necessary toreplace all of the printing equipment. This represents a considerablecost to users of such equipment.

[0040] It is an object of the present invention to obviate or mitigatethe problems outlined above.

[0041] According to a second aspect of the present invention, there isprovided a transfer printer for transferring ink from a printer ribbonto a substrate which is transported along a predetermined path adjacentthe printer, comprising a printer ribbon drive mechanism fortransporting ribbon between first and second ribbon spools, a printheadarranged to contact one side of the ribbon to press an opposite side ofthe ribbon into contact with a substrate on the predetermined path, aprinthead drive mechanism for transporting the printhead along a trackextending generally parallel to the predetermined substrate transportpath and for displacing the printhead into and out of contact with theribbon, and a controller controlling the printer ribbon and printheaddrive mechanisms, the controller being selectively programmable eitherto cause the ribbon to be transported relative to the predeterminedsubstrate transport path with the printhead stationary and displacedinto contact with the ribbon during printing, or to cause the printheadto be transported relative to the ribbon and the predetermined substratetransport path and to be displaced into contact with the ribbon duringprinting.

[0042] Thus the second aspect of the present invention provides aprinting apparatus with sufficient versatility to be able to be used inboth continuous and intermittent applications.

[0043] The transfer printer of the second aspect of the invention asdefined above may be used in conjunction with any or all of the featuresof the first aspect of the present invention as discussed above. Inparticular, the drive mechanism may be bidirectional, the drivemechanism may comprise two stepper motors each driving a respective oneof the first and second ribbon spools in the direction of tapetransport, ribbon tension may be monitored and the stepper motorscontrolled to maintain the monitored tension within predeterminedlimits, the printhead drive mechanism may comprise a further steppermotor coupled to the printhead, and the printhead may be mounted on acarriage that is displaceable along a track. In addition, first andsecond carriages which are interchangeable may be provided to enableprinting on a substrate travelling in either direction along thesubstrate transport path and a peel off roller mounted adjacent theprinthead may be reversible in position relative to the printhead.

[0044] As outlined above, in tape drives which are used to transfer tapesuch as a printer ribbon between two spools, the diameters of the spoolschange during the course of tape transfer from one spool to the other.This dramatically affects the relative speeds of the two spools whichmust be maintained if the tape is to remain in tension. Various attemptshave been made to account for this effect, and notably the approachadopted in U.S. Pat. No. 4,479,081. None of the known approaches howeveris satisfactory in delivering a reliable accurate measurement of spooldiameters to thereby enable an accurate and appropriate control of drivemotor speeds in an arrangement in which the two motors are operating inpush-pull mode. Whereas some of the known systems can cope with tapedrives in which the initial conditions are always the same (for examplea fresh supply spool of known outside diameter is connected to an emptytake-up spool), in many applications it is quite often the case that anoperator will fit to a machine a tape which has been partially used suchthat the supply spool which initially was of known outside diameter haspartly been transferred to the take-up spool.

[0045] It is an object of the present invention to obviate or mitigatethe problems outlined above.

[0046] According to a third aspect of the present invention, there isprovided an apparatus for measuring the diameters of two spools of tapemounted on a tape drive mechanism which is drivable to transport tapebetween the spools, comprising an optical sensing system including atleast one light emitter and at least one light detector arranged suchthat an optical path is established therebetween, a transport mechanismsupporting at least part of the optical sensing system and drivable soas to cause the optical path to sweep across a space within which spoolsto be measured will be located, and a controller operative to controlthe transport mechanism, to detect positions of the transport mechanismin which the output of the detector changes to indicate a transitionbetween two conditions in one of which the optical path is obstructed bya spool and in the other of which the optical path is not obstructed bythat spool, and to calculate the spool diameters from the detectedpositions of the transport mechanism in which the detector outputchanges.

[0047] This third aspect of the present invention makes it possible toaccurately determine spool sizes. In an apparatus such as a transferprinter with a displaceable printhead the displaceable component can bereadily mounted on the displaceable printhead so as to require noadditional electromechanical components over and above those necessaryfor the normal functioning of the apparatus.

[0048] The apparatus of the third aspect of the present invention asdefined above may be used in conjunction with any of the features of thefirst and second aspects of the invention as defined above.

[0049] Printheads used in for example transfer printers must beaccurately positioned relative to a platen which supports a substrate tobe printed if good quality print is to be produced, particularly at highprinting speeds. An angular displacement of only a few degrees canradically affect print quality. The traditional approach to dealing withthis problem is to position a printhead on an appropriate supportassembly in a nominally correct position, to then run a test print tosee what quality results, and to then mechanically adjust the positionof the printhead so as to optimise print quality. This involves aninstaller making very small mechanical adjustments using for examplespacers. This can be a time consuming process.

[0050] It is an object of the present invention to obviate or mitigatethe problems outlined immediately above.

[0051] According to a fourth aspect of the present invention, there isprovided a printing apparatus comprising a housing, a printhead mountedon a printhead support assembly which is displaceable relative to thehousing in a direction parallel to a print ribbon path, a first drivemechanism for displacing the printhead support relative to the housing,a roller which in use supports a substrate to be printed on the side ofthe ribbon path remote from the printhead, a second drive mechanism fordisplacing the printhead relative to the printhead support assembly to aprinting position in which a portion of the printhead bears against theroller or any substrate or ribbon interposed between the printhead androller, and a controller for adjusting the first drive mechanism toadjust the angular position of the printhead relative to the rotationaxis of the roller.

[0052] Preferably, the portion of the printhead that bears against theroller or any substrate or ribbon interposed between the printhead androller, is the portion of the printhead that contains selectivelyenergisable printing elements. The elements may be linearly arrangedalong the portion of the printhead, for example the linear array ofelements may arranged along an edge, or parallel in close proximity toan edge of the printhead.

[0053] In operation, an installer could initially position a printheadso that it would assume a nominal position which would be expected toproduce good quality print. A test print run would then be used toassess print quality, the printhead support would then be displacedrelative to the housing, and a fresh print run would be conducted, theprocess being repeated until the resultant print quality was optimised.There is thus no requirement for the installer to make small mechanicaladjustments to the position of the printhead on its support.

[0054] The printing apparatus in accordance with the fourth aspect ofthe present invention may be used in conjunction with any of thefeatures of the first, second and third aspects of the invention asdefined above.

[0055] In many tape drive mechanisms and particularly in ribbon printingmachines, loading a fresh print ribbon can be a difficult process as theprint ribbon has to be correctly positioned along a non-linear path.Often replacement print ribbons are mounted in a cassette which isdesigned to be readily mounted in a predetermined orientation on anassociated printing apparatus. In such arrangement it is generallynecessary to position a length of ribbon extending between support onthe cassette between a printhead and a peel off roller. This isdifficult to achieve unless the printhead and peel off roller can bemoved apart to provide a wide enough track into which the ribbon can beinserted.

[0056] It is known to provide an arrangement in which either theprinthead or the peel off roller can be displaced by a lever mechanismwhich is actuated when a cassette is mounted on a printing apparatus.For example if the cassette is held in position by a mechanical latch,release of the latch will move the printhead and peel off roller apartwhereas engagement of the latch moves them together to a ready-to-printposition.

[0057] Such arrangements are satisfactory in terms of performance butdisadvantageous as valuable space is occupied by the lever mechanisms,thereby reducing the space available for taking large diameter spools oftape.

[0058] It is an object of the present invention to obviate or mitigatethe problems outlined above.

[0059] According to a fifth aspect of the present invention, there isprovided a printing apparatus comprising a housing, a printhead mountedon a printhead support assembly which is displaceable relative to thehousing in a direction parallel to a print ribbon path, a first drivemechanism for displacing the printhead support relative to the housing,a peel off roller mounted on the printhead support assembly anddisplaceable with the printhead in the said parallel direction, and asecond drive mechanism for displacing the printhead relative to theprinthead support assembly and peel off roller between a ready to printposition adjacent the print ribbon path and a printing position in whichthe printhead would contact a print ribbon on the path, wherein a cammechanism is provided which is engaged as a result of displacement ofthe printhead support assembly to a predetermined position and whenengaged causes retraction of the printhead away from the ready to printposition to a position spaced from the peel roller and the print ribbonpath.

[0060] In an arrangement in accordance with the fifth aspect of thepresent invention, when a cassette carrying a print ribbon is to bereplaced, an electronic signal can be generated to cause transport ofthe print support assembly to a predetermined position (a “docked”position). This automatically retracts the printhead away from thepeel-off roller, enabling the easy insertion of a tape between theprinthead and peel roller.

[0061] The printing apparatus according to the fifth aspect of thepresent invention may be used in conjunction with any of the features ofthe first, second, third and fourth aspects of the present invention asdefined above.

[0062] Another problem encountered with printing machines is that ofachieving sufficient tape transport speeds in the interval betweenprinting phases of successive printing cycles. In some instances thetime taken to transport a printing ribbon by a distance equal to thelength of ribbon traversed by the printing head during one printingcycle is a limiting factor in overall machine speed. It would beadvantageous to be able to reduce the distance that a ribbon is advancedbetween any two successive printing cycles.

[0063] According to a sixth aspect of the present invention, there isprovided a printing apparatus comprising a printhead, a printing ribbondrive mechanism for advancing a printing ribbon between the printheadand a path along which in use a substrate to be printed is advanced,means for applying the printhead to a ribbon supported in the drivemechanism, the printhead comprising an array of printing elements eachof which may be selectively energised to release ink from a portion ofribbon- contacted by that element, and a controller for controllingenergisation of the printing elements and the advance of the ribbon soas to perform a series of printing cycles each of which includes aprinting phase during which relative movement between the printhead andribbon results in the printhead reversing a predetermined length ofribbon and a non-printing phase during which the ribbon is advanced apredetermined distance relative to the printhead, herein the controlleris arranged selectively to energise different groups of printingelements during successive printing cycles, the groups of elements beingdistributed on the printhead such that different groups contactdifferent portions of the ribbon, and the controller is arranged toadvance the ribbon such that the said predetermined distance of ribbonadvance is less than the said predetermined length of ribbon, the groupsof printing elements being energised such that the ribbon is advanced byat least said predetermined length of ribbon in the interval between anytwo printing phases in which the same group of printing elements areenergised.

[0064] If the printing elements are arranged in two groups, for examplealternate pixels distributed across a linear printing head, an image maybe printed in one printing cycle using one group, the ribbon may beadvanced by half the length of ribbon traversed by the printer duringthe first cycle, a second image may be printed using the other groupduring a second cycle, the ribbon may again be advanced by half thetraverse distance of the printing head, and then the first group may beenergised during a third printing cycle and so on. Thus the maximum tapetravel between successive printing cycles can be half that which must beaccommodated in conventional printing systems.

[0065] The printing apparatus according to the sixth aspect of thepresent invention may be used in conjunction with any of the features ofthe first, second, third, fourth and fifth aspects of the presentinvention as defined above.

[0066] It will of course be appreciated that if the printing elementswere divided into three groups, tape advance between successive cyclescould be limited to one third of the length of ribbon traversed by theprinthead in one cycle.

[0067] An embodiment of the present invention will now be described, byway of example, with reference to the accompanying drawings, in which:

[0068]FIG. 1 is a schematic illustration of a printer ribbon drivesystem in accordance with the present invention;

[0069]FIG. 1a is an illustration of a modification to the drive systemof FIG. 1;

[0070]FIG. 2 is a perspective view of a printer drive assembly of aribbon drive system as described with reference to FIG. 1.

[0071]FIG. 3 is a schematic perspective view of a printer ribboncassette which maybe mounted on the assembly of FIG. 2;

[0072] FIGS. 4 to 9 are further illustrations of the drive assembly ofFIG. 2;

[0073]FIG. 10 is a perspective view of a printhead support carriageincorporated in the drive assembly of FIG. 2;

[0074]FIG. 11 is a perspective view of an alternative printhead supportcarriage to that shown in FIG. 10 which may be used to reverse thepositions of components of the drive assembly of FIG. 2;

[0075]FIG. 12 is a view of the drive assembly after conversion using thealternative printhead support of FIG. 11;

[0076] FIGS. 13 to 16 illustrate the use of interleaved printing usingthe drive assembly of FIG. 2;

[0077]FIG. 17 schematically illustrates the operation of an opticalprinter ribbon spool diameter measuring system;

[0078]FIG. 18 is a schematic illustration of a circuit for monitoringthe power consumed by steppers motors incorporated in the drive assemblyof FIG. 2;

[0079]FIG. 19 is a schematic illustration of a circuit for monitoringthe charging ratio between the diameters of ribbon spools mounted on thedrive assembly of FIG. 2;

[0080]FIG. 20 illustrates an alternative approach to monitoring ribbonspool diameters;

[0081]FIG. 21 illustrates the adjustment of printhead angle inaccordance with the invention; and

[0082]FIG. 22 illustrates the use of apparatus in accordance with thepresent invention to produce images whilst relying upon only limitedprinter ribbon advance.

[0083] Referring to FIG. 1, the schematically illustrated printer inaccordance with the present invention has a housing represented bybroken line 1 supporting a first shaft 2 and a second shaft 3. Adisplaceable print head 4 is also mounted on the housing, the print headbeing displaceable along a linear track as indicated by arrows 5. Aprinter ribbon 6 extends from a spool 7 received on a mandrel 8 which isdriven by the shaft 2 around rollers 9 and 10 to a second spool 11supported on a mandrel 12 which is driven by the shaft 3. The pathfollowed by the ribbon 6 between the rollers 9 and 10 passes in front ofthe print head 4. A substrate 13 upon which print is to be depositedfollows a parallel path to the ribbon 6 between rollers 9 and 10, theribbon 6 being interposed between the print head 4 and the substrate 13.

[0084] The shaft 2 is driven by a stepper motor 14 and the shaft 3 isdriven by a stepper motor 15. A further stepper motor 16 controls theposition on its linear track of the print head 4. A controller 17controls each of the three stepper motors 14, 15 and 16 as described ingreater detail below, the stepper motors being capable of driving theprint ribbon 6 in both directions as indicated by arrow 18.

[0085] In the configuration illustrated in FIG. 1, the spools 7 and 11are wound in the same sense as one another and thus rotate in the samerotational direction to transport the tape. FIG. 1a illustrates amodification of the drive system of FIG. 1 in which the spools are woundin the opposite sense to one another and must rotate in oppositedirections to transport the tape. Thus the first spool 7 rotatesclockwise whilst the second spool 11 rotates anticlockwise to transportthe printer ribbon 6 from the first spool 7 to the second spool 11.

[0086] As described in greater detail below, the printer schematicallyillustrated in FIG. 1 can be used for both continuous and intermittentprinting applications. In continuous applications, the substrate 13 willbe moving continuously. During a printing cycle, the print head will bestationary but the ribbon will move so as to present fresh ribbon to theprint head as the cycle progresses. In contrast, in intermittentapplications, the substrate is stationary during each printing cycle,the necessary relative movement between the substrate and the print headbeing achieved by moving the print head during the printing cycle. Theribbon generally will be stationary during the printing cycle. In bothapplications, it is necessary to be able to rapidly advance and returnthe ribbon 6 between printing cycles so as to present fresh ribbon tothe print head and to minimise ribbon wastage. Given the speed at whichprinting machines operate, and that fresh ribbon should be presentbetween the print head and substrate during any printing cycle, it isnecessary to be able to accelerate the ribbon 6 in both directions at ahigh rate and to accurately position the ribbon relative to the printhead. In the arrangement shown in FIG. 1 it is assumed that thesubstrate 13 will move only to the right as indicated by arrow 19 but asdescribed below the apparatus can be readily adapted to print on asubstrate travelling to the left in FIG. 1.

[0087] Referring to FIGS. 2, 3 and 4, electromechanical componentsmaking up the printer described-in outline with reference to theschematic illustration of FIG. 1 will now be described. The printerhousing 1 comprises a casing 20 in which various electronic componentsto be described below are housed behind a cover plate 21. The shafts 2and 3 project through apertures in the cover plate 21, guide pins 9 aand 10 a project from the cover plate 21, and the print head 4 ismounted above the cover plate 21. The print head 4 is part of anassembly which is displaceable along a linear track 22 which is fixed inposition relative to the cover plate 21. The stepper motor 16 whichcontrols the position of the print head assembly is located behind thecover plate 21 but drives a pulley wheel 23 that in turn drives a belt24 extending around a further pulley wheel 25, the belt being secured tothe print head assembly. Thus rotation of the pulley 23 in the clockwisedirection in FIG. 4 drives the print head assembly to the left in FIG. 4whereas rotation of the pulley 23 in the anti-clockwise direction inFIG. 4 drives the print head assembly to the right in FIG. 4. The pulleywheels 23 and 25 and the linear track 22 are mounted on a rigid bracket26 which extends upwardly from the cover plate 21. FIG. 2 shows drivediscs mounted on the shafts 2 and 3, the drive discs definingdiametrically spaced sockets intended to engage ribbon spools 8 and 12,whereas in FIG. 4 the drive discs have been removed to show the uppersurfaces of the stepper motors 14 and 15.

[0088] Referring to FIG. 3, this illustrates a printer ribbon supportedon a cassette which may be mounted on the printer of FIG. 2. Hollowroller supports 9 b and 10 b are intended to receive the pins 9 a and 10a respectively shown in FIG. 2, such that the combination of pin 9 a andhollow roller 9 b together constitute the roller 9 of FIG. 1 and suchthat the pin 10 a and hollow roller 10 b together constitute the roller10 of FIG. 1. The spools 7 and 11 are supported on the mandrels 8 and 12which are a push fit on rotatable shafts mounted on a common cover platewith the hollow rollers 9 b and 10 b. The rotatable shafts define pinswhich engage with the sockets defined on the drive discs driven by theshafts 2, 3. Thus, with the cassette in place, the ribbon can betransferred between the two spools 7 and 11.

[0089] The housing cover plate 21 FIG. 2) also supports an upstandingrear bracket 27 on which a pair of emitters 28, 29 are supported. Thesetwo emitters operate in cooperation with a receiver which isdisplaceable with the print head assembly as described in greater detailbelow.

[0090] The print head assembly 4 is shown in a “docked” position inFIGS. 2 and 4 and in a position in FIG. 5 in which it is ready to printon a roller platen 30 (assuming operation in a continuous mode with acontinuously moving substrate), and in a ready to print position in FIG.6 in which the print head is ready to print on a substrate which isstationary and positioned in front of a stationary flat platen 3 1. Inthe position shown in FIGS. 2 and 4, an edge 32 of the print head 4 isretracted behind the ribbon path between rollers 9 and 10 whereas apeel-off roller 33 is positioned on the opposite side of the ribbon pathfrom the print head 4. This makes it an easy matter to install a freshcassette of ribbon. In contrast, in the ready-to-print positions shownin FIGS. 5 and 6, the print head 4 has been advanced so that the edge 32projects just beyond the outer extremity of the roller 33. Thus in theready-to-print position the print ribbon passes around the edge 32 andis deflected away from the underlying substrate by the roller 33.

[0091] The edge 32 of the print head 4 (which is of conventional form)supports an array of heating elements each of which is selectivelyenergiseable. When the ribbon 6 is sandwiched between the head 4 and asubstrate 13, ink adjacent any energised heating element is melted andtransferred to the substrate. Thus by appropriate control of the heatingelements, small portions of ink carried by the ribbon 6 can betransferred to the substrate 13. Each of these portions of ink can beconsidered as defining one pixel of the image to be printed.

[0092] Referring now to all of FIGS. 2 to 9, the detailed structure ofthe print head assembly and the slider upon which it is mounted will bedescribed. FIG. 9 shows the print head assembly pulled forward to anadjustment position revealing associated components of the assembly.FIG. 9 is the best view of a slot 34 formed in the upstanding bracket 26on which the linear track 22 is mounted. A slider 35 supporting a printhead carriage 36 is mounted on the linear track 22. The slider 35 andtrack 22 are high-accuracy products to provide smooth, low friction,parallel movement of the print head carriage 36 relative to the bracket26. An optical detector 37 is mounted on the print head carriage 36 soas to be in register with the slot 34 formed in the bracket 26. Thedetector 37 is used as described below to detect light emitted from theemitters 28 and 29, the slot 34 ensuring that the only obstructionbetween the detector 27 and the emitters 28 and 29 will be any spools ofribbon mounted on the printer in a cassette such as that illustrated inFIG. 3. The cassette is secured against displacement relative to thecomponents illustrated in FIG. 3 by a permanent magnet (not shown)incorporated in the cassette body and cooperating with a circular steelkeeper 38 mounted on top of the bracket 26. Alternative arrangements forsecuring the cassette in position are of course possible, for examplemechanical latch assemblies.

[0093] The print head carriage 36 supports the print head assembly whichcomprises the print head 4 which is bolted to a pivotal plate 39 that ismounted to pivot about pivot pin 40 that in turn is mounted on a plate41 bolted to the print head carriage 36. A spring 42 biases the plate 39towards the plate 41 so that in the absence of any obstruction the printhead 4 will assume the position relative to the print head carriage 36as shown in FIG. 4. The peel off roller 33 is fixed in position on anarm 43 which is bolted to the print head carriage 36.

[0094] A pneumatic drive unit 44 is a sliding fit within a slot providedin the print head carriage 36 and drives a piston 45 which is shown inthe extended position in FIG. 8 and the retracted position in FIG. 7.The pneumatic drive 44 is connected to a flexible pneumatic supply line(not shown) connected to an air inlet 46 (FIG. 2). The inlet 46 isconnected to a tube 47 which extends through an opening in the printhead carriage 36 so as to communicate with the pneumatic drive unit 44.The pneumatic drive unit piston 45 bears against a U-shaped member 48which is coupled by pivot pin 49 to a U-shaped bracket 50. The bracket50 supports a pin 51 (FIG. 9) which is intended to engage in a slot 52in a cam plate 53. The bracket 50 defines a curved comer 54 which isintended to engage against curved surface 55 defined in plate 39 asshown in FIGS. 7 and 8. If however the pin 51 is received in and pushedto the blind end of the slot 52, the bracket 50 is pushed away from theprint head 4, enabling the plate 39 to swing towards the plate 41 sothat the print head 4 assumes the docked position shown in FIGS. 2 and4.

[0095] The bracket 50 is spring biased by a spring (not shown) coupledto a lever 50a (see FIG. 7) so as normally to assume the position shownin FIG. 7. If pressurised air is then supplied to the pneumatic drive44, the assembly assumes the position shown in FIG. 8 in which it willbe seen that the printing edge 32 of the print head 42 has been pushedwell beyond the peel-off roller 33. If with the pneumatic drive unit 44de-energised and therefore the U-shaped member 48 in the position shownin FIG. 7 the carriage is moved so that the pin 51 enters the slot 52,further movement of the carriage in the same direction will cause thepin 51 to move into the blind end of the slot, thereby causing thebracket 50 to turn about the pivot pin 49 so as no longer to obstructmovement of the print head 4 to the docked position. If movement of thecarriage is then reversed, the pin 51 causes the bracket 50 to swing outagain, pushing the print head 4 to the position shown in FIG. 7. Theposition shown in FIG. 7 corresponds to “ready to print” and theposition shown in FIG. 8 corresponds to “printing”.

[0096]FIG. 10 is a perspective view of the print head carriage 36showing the socket which in the assembled apparatus receives thepneumatic drive unit 44. An opening 56 is provided to receive the airinlet tube 47 (see FIG. 7). A tongue 57 projects from the lower edge ofthe print head carriage 36 and is used in a manner not shown to attachthe print head carriage to the belt 24.

[0097] In the embodiment of the invention as illustrated in FIGS. 1 to10, it is intended that a substrate to be printed travels past the printhead in the left to right direction with respect to FIG. 5 or the printhead when printing travels in the right to left direction with respectto the platen 31 in FIG. 35. The peel-off roller 33 is in all instancespositioned on the downstream side of the printing edge 32. There aremany circumstances however where such an arrangement is not convenientand it would be desirable to reverse the arrangement so that therelative positions of the edge 32 and the peel off roller 33 arereversed and the disposition of the print head 4 is also reversed. Thiscan readily be achieved with the illustrated apparatus by replacing theprint head carriage 36 shown in FIG. 10 with the print head carriage 58shown in FIG. 11. FIG. 12 illustrates the resultant assembly. It will benoted that the print head carriage 58 of FIG. 11 also defines a socket59 for receiving the pneumatic drive unit 44 and an opening 60 forreceiving the air inlet tube 47. It will also be noted that the printhead carriage 58 of FIG. 11 is a mirror image about a vertical plane ofthe print head carriage 36 of FIG. 10.

[0098] Referring to FIG. 12, it will be seen that in addition toreversing the position of the print head 4 and the peel off roller 33,the cam plate 53 has also been rotated through 180° and fitted on theopposite side of the magnet 38 to its position in the embodiment ofFIGS. 1 to 10. The arm 43 on which the peel off roller 33 is mounted hasalso been moved so as to continue to be located adjacent the cover plate21.

[0099] The described printer arrangement provides a number of verysignificant advantages. Firstly, it is possible to use the sameapparatus for both continuous and intermittent printing. Conversion of aproduction line from one form of printing to another does not thereforemean that new printers must be purchased. Secondly, by making relativelyminor modifications involving only one additional component (thealternative print head carriages of FIGS. 10 and 11) the same apparatuscan be used for both left hand and right hand applications, using theseterms in the sense of FIG. 2 (left hand) and FIG. 12 (right hand).Thirdly, ribbon replacement is a simple matter given that when in thedocked position the print head 4 is automatically pulled back away fromthe peel roller 33 so as to provide a wide track into which areplacement-printer ribbon carried on a cassette can be inserted.

[0100] Referring to FIGS. 13, 14, 15 and 16, different methods of makingefficient use of the printer ribbon using the apparatus described inFIGS. 1 to 12 will be described. All of these methods rely upon the highaccuracy within which ribbon can be delivered to the print head so as tominimise ribbon wastage.

[0101] Referring to FIG. 13, this is a view of a ribbon the length ofwhich is indicated by arrow 61 and with which six individual printingoperations have been performed using overlapping regions of the ribbon.These six regions are indicated as regions 62 to 67, the second half ofregion 62 overlapping with the first half of region 63, the second halfof region 63 overlapping with the first half of region 64 and so on.Assuming printing on a substrate, the region 62 is printed, the ribbonis then advanced by half the length of the regions, the region 63 isprinted, the ribbon is then again advanced by half the length of theregions, the region 64 is then printed and so on.

[0102] Such overlapping printed regions could be used in both continuousand intermittent printing processes. In the described arrangement,adjacent regions overlap by half the width of each region, but differentproportions of overlap could be envisaged. Given that adjacent printingregions overlap, it is important that a region of the ribbon which isoverlapped by two adjacent printing regions is used in a manner whichensures that printing progresses only on the basis of using portions ofthe ribbon which are used in only one of the two overlapping regions.This can be achieved for example-by selecting only alternate portions ofthe ribbon within any one printing region. For example, as illustratedin FIG. 14, if adjacent heating (pixel) elements on the printing headare represented by ribbon areas 68 and 69, ribbon areas 68 would be usedin printing one region (for example region 62) and ribbon areas 69 wouldbe used in printing the adjacent region (region 63). In this manner,providing the spacing between adjacent pixels on the print head is smallenough to enable an image of reasonable quality to be printed using onlyalternate pixels, twice the number of images can be generated from aribbon than would be the case if all the pixel elements were used forprinting purposes in a single image and there was no overlap betweenprinting regions. In addition however the distance that the ribbon mustbe advanced between printing phases in successive printing cycles isreduced by half. This is advantageous as in some applications thisenables faster machine operation.

[0103] To illustrate this advantage, FIG. 15 shows conventional printingonto a substrate with no overlap between successive cycles whereas FIG.16 illustrates the same operation relying upon such overlap.

[0104] Referring to FIG. 15, a substrate 70 is shown on which successiveimages 71 and 72 have been printed. Shown beneath the substrate is aprint ribbon 73 on which areas 74 and 75 have been used to produce theimages 71 and 72. The ribbon transport length is indicated by the arrow76 and is equal to twice the length of a single image.

[0105] Referring to FIG. 16, this shows how overlapping printing canboth reduce ribbon usage and reduce the distance of ribbon transportbetween successive printing phases. It will be seen that each of theareas 74 and 75 in FIG. 16 is only half the length of the correspondingareas in FIG. 15 and the ribbon transport distance is therefore halved.In some applications, where rapid ribbon transport is required, halvingthe distance that ribbon must be transported between successive printingphases can significantly improve the ability of the device to operate athigh speed. It will also be appreciated that more than two groups ofprinting elements may be used so that in the case of for example threegroups the length of required ribbon transport would be only one thirdof the image length. Thus there would be a trade off between printerribbon transport length and image quality but this aspect of the presentinvention does give the operator of such equipment increased flexibilitywhich in some applications will be of real economic significance.

[0106] The advantages described with references to FIGS. 13 to 16 canonly be achieved if the print ribbon can be positioned relative to thesubstrate and the print head with great accuracy. The conventionalapproach to achieving accurate control of tape acceleration,deceleration, speed and position has relied upon a capstan rollerpositioned between feed and supply spools, but the present inventionrelies upon a completely different approach, that is the accuratecontrol of the drive applied to the stepper motors 14 and 15 (FIG. 1)which drive the ribbon spools. The stepper motors operate in push-pullbi-directional mode, that is if the tape is travelling in one directionbetween the spools both stepper motors are driven in that direction, andconversely when the ribbon is being driven in the opposition directionboth stepper motors are driven in that opposite direction. Coordinationof the drive to the two stepper motors requires knowledge of thediameters of the spools and this is achieved using the light emittingdevices 28 and 29 and the light detecting device 37 as shown in forexample FIG. 2.

[0107]FIG. 17 illustrates how the light emitting devices 28 and 29 andthe detector 37 are used to determine the spool diameters. The detector37 is mounted on the print head carriage 36 and is displaceable betweenthe position indicated by line 76 and the position indicated by line 77.As the detector 37 is moved to the right in FIG. 17 from the positionindicated by line 76, initially emitter 28 is energised. Initially thedetector 37 is in the shadow cast by spool 7, but as soon as thedetector 37 crosses the plane indicated by line 78 a an output will begenerated. That output will disappear as the detector 37 crosses theplane indicated by line 78 b. The detector 37 is then advanced to theposition indicated by line 77 and then returned after the emitter 28 hasbeen de-energised and the emitter 29 has been energised. Initially thedetector 37 will be in the shadow of spool 11 but will generate anoutput as soon as it reaches the plane indicated by the line 79 a. Thatoutput will disappear as the detector 37 crosses the plane indicated bythe line 79 b. The positions relative to the detector displacement atwhich the detector 37 intersects the planes 78 a, 78 b, 79 a and 79 bcan thus be determined. The dimension A, that is the distance betweenthe rotation axes of the two spools, is known. The perpendiculardistance B between the track followed by the detector 37 and the planein which the emitters 28 and 29 are located is known, as is theperpendicular distance C from the axes of the shafts 2 and 3 to thetrack followed by the detector 37. From these dimensions the diametersD1 and D2 of spools 7 and 11 can be derived using simple trigonometry.

[0108] Two emitters 28, 29 are used to ensure that for any one spool thedetector 37 can “see” the shadow cast by at least one of the emittersregardless of spool diameter size. It will be appreciated however thatother dispositions of one or more emitters and one or more detectorscould be envisaged.

[0109] It will be appreciated that the calculation of the spooldiameters would be somewhat simpler if the planes 78 a, 78 b, 79 a and79 b were perpendicular to the direction of displacement of the detector37. This can be achieved by for example replacing the emitters 28 and 29with a mirror extending parallel to the direction of displacement of theprint head carrier 36 and arranging both a transmitter and a detector onthe print head carriage 36, the detector detecting light only when bothit and the emitter are on a plane perpendicular to the mirror. Althoughsuch an arrangement is simple in terms of the required trigonometry ithas disadvantages in that a failure of either the transmitter ordetector could be interpreted as the detector being in the shadow of oneof the spools.

[0110] Given knowledge of the spool diameters, the spools can be drivenin push-pull mode so as to achieve high rates of acceleration anddeceleration by appropriate control of the speeds of rotation of the twostepper motors. Tension in the ribbon between the two spools musthowever by closely controlled to avoid the tension becoming too high(resulting in over tightening of the ribbon on the spools or even ribbonbreakage) or the tension becoming too low (resulting in loss ofpositional control as a result of the ribbon becoming slack). To avoidthis occurring, changes in spool diameters over time are monitored byreference to the stepper motors and tension in the ribbon is directlymonitored by reference to the current drawn by the stepper motors.

[0111] In one embodiment of the invention, when a fresh cassette isfitted on to an apparatus such as that described with reference to FIGS.1 to 10, one of the cassette shafts will support an almost empty spool(the take up spool) and the other will support an almost full spool (thesupply spool). The steppermotor associated with the take up spool willbe referred to below as the take up motor and the other stepper motorwill be referred to as the supply motor.

[0112] Initially the take up motor is energised to remove any slack fromthe length of ribbon extending between the two spools. A print head scanis then conducted with the optical system described with reference toFIG. 17 to obtain an initial estimate of the diameters of the spools.The supply motor is then energised in order to tension the ribbonextending around the supply spool. The take up motor is then driven soas to draw ribbon from the supply spool, the supply spool beingdeenergised The number of steps taken by the motor driving the take-upspool is monitored. The other motor is not stopped, but generates aback-emf resulting in the generation of pulses that are counted. After afew turns of the spools the number of steps taken by the take-up motorand the number of pules generated by the supply spool motor are countedand the counted numbers are used to establish the ratio between the twodiameters. The ribbon is then brought to a controlled halt. Both motorsare decelerated in a controlled manner to avoid overrun. Thus the supplyspool motor is driven by pulses to cause deceleration. The applicationof deceleration pulses to the supply spool motor in synchronism withmotor rotation is achieved by monitoring the back-emf generated in onewinding of that motor, and then energising that winding at anappropriate time to apply a decelerating torque. A number of rotationsof the take up spool are required to minimise the chance of any tails ofribbon extending from the spools obstructing the optical paths of thescanning arrangement as illustrated in FIG. 17. A further optical scanis then performed in both directions to determine the radius of the takeup spool whilst that spool is stationary. An optical scan is thenrepeated as the spool is rotated in 30° increments around the steppermotor shaft by stepping the motor by the appropriate number of steps,that number being a constant. This builds up a map of the dimensions ofthe spool (which may not be perfectly circular) and this map is used tocalculate the average radius for each spool for the arc that each willrotate in each ribbon feed and further use these radii to calculatevariations in diameter around the spool axes. This makes it possible toaccurately determine the circumference of each spool and the effect of apredetermined number of steps advance of the motor driving that spool.For example the different calculated radii can be used to calculate thestep rate and the number of steps required by each motor to drive thespools in an appropriate manner so as to feed the ribbon a predetermineddistance. These radii and step rates may then be used in tensionmonitoring calculations such as those described below.

[0113] The same optical scan procedure is then performed in bothdirections to measure the radius of the supply spool. This informationis then combined with the previously calculated ratio of spool diametersto give an accurate set of data related to the spool diameters andshapes. Ribbon fed from the supply spool to the take up spool is thenrewound back on to the supply spool so as to avoid ribbon wastage.

[0114] Stepper motors generally comprise two quadrature-wound coils andcurrent is supplied in a sequence of pulses to one or both of the coilsand in both senses (positive and negative) so as to achieve step advanceof the motor shafts. In order to achieve a reasonable performancedespite the inherent electrical time constant of these coils it is wellknown to over-drive stepper motors by applying a voltage that is muchlarger than the nominal rating of the motor and to pulse width modulatethis voltage when the desired motor current is reached. For example,with a 3.6 volt motor capable of taking say 2 amps, a voltage of 36volts may be applied. This results in a very rapid rise in currentthrough the motor, typically in a few tens of micro seconds. Given suchoverdriving of the supply voltage, relatively short periods of supplyvoltage application are separated by relatively long periods duringwhich no supply voltage is applied. As a result current from the supplyto the motors is very far from smooth. In addition, even when a motor isoperating with zero load relating to the function that it performs(equating to zero tension in the printer ribbon), the supply current tothe motor will be a function of various factors such as the speed ofrotation of the motor, the particular characteristics of that motor(efficiency etc.), and the particular characteristics of the motor drivecircuitry (gain and offset variances). It is necessary therefore tocalibrate the motors to take account of current variation related tothese factors rather than motor load.

[0115] The motors are calibrated by driving each of them in zero-loadconditions at each of a series of different speeds, for example atspeeds corresponding to 125 steps per seconds, 250 steps per second, 375steps per second and so on in increments of 125 steps per seconds up to5000 steps per second. This will generally cover the range of ribbonspeeds required for ribbon advancement, that range generally being from100 mm per second to 600 mm per second ribbon transport speed. Thisprocess is repeated a number of times, for example twenty times, and theaverage result is used to calculate a motor calibration factor x foreach step rate, and for each motor. The following relationship is used:

x=N/V

[0116] where

[0117] x is the calibration factor for the motor at the given step rate.

[0118] V is the average measured motor operation value at the given steprate.

[0119] N is a constant normalisation or scaling factor.

[0120] From the above for each motor a series of values x is calculatedfor each of the predetermined step rates. When the apparatus is in use,for a given step rate one of the values x is selected for use in thecalculation of ribbon tension, or a value for x is calculated for thegiven step rate by interpolation from the two values of x for thepredetermined step rates closest to the given rate.

[0121]FIG. 18 illustrates the calculation of the values V both duringmotor calibration and in subsequent ribbon tension control. Referring toFIG. 18, a regulated power supply 80 energises a first motor drivecircuit 81 and a second motor drive circuit 82. Current from the supply80 to the motor drive circuit 81 is delivered through a low resistanceresistor 83, the potential developed across the resistor 83 beingapplied to a level translator 84. Similarly, current to the motor drive82 is delivered through a low resistance value resistor 85 and thevoltage developed across that resistor is applied to a level translator86. The outputs of the level translators 84 and 86 are applied toanalogue to digital converters 87 and 88 the outputs of which areapplied to a micro controller 89. The micro controller delivers a pulsedoutput 90 to the first motor drive 81 and a pulsed output 91 to thesecond motor drive 82. The motor drives energise stepper motorsschematically represented by cylinders 92 and 93 which drive respectivespools 94 and 95.

[0122] During motor calibration, no spools are mounted on the outputs ofthe stepper motors 92 and 93. For a given step rate for each motor theoutputs of the ADC's 87 and 88 are recorded such that x and V for eachmotor at each of the preselected step rates is known. Those values arethen used as described below to enable direct monitoring of ribbontension in the ribbon between the spools 94 and 95, these spools havingbeen mounted on the output shafts of the stepper motors 92 and 93.

[0123] The formulas used for tension calculation are as follows,assuming that motor 92 is pulling and motor 93 is pushing:

V ₁ x ₁=(N+r ₁ tx ₁)f(T)   (1)

V ₂ x ₂=(N−r ₂ tx ₂)f(T)   (2)

[0124] Where:

[0125] V₁ is the output of ADC 88 given a selected constant step-rateribbon feed

[0126] V₂ is the output of ADC 87 during ribbon feed

[0127] r₁ is the radius of the spool 94

[0128] r₂ is the radius of the spool 95

[0129] x₁ is the calibration factor for motor 92 for the selectedconstant step rate

[0130] x₂ is the calibration factor for motor 93 for the step rate ofmotor 93

[0131] N is the scaling factor used during motor calibration

[0132] t is the ribbon tension

[0133] f(T) is a temperature-related function

[0134] Temperature variations which will affect the measured values V₁and V₂ will generally affect both motors to the same extent. Thereforeby dividing equation (1) by equation (2) the functions f(T) will cancelout. The equation can therefore be resolved to derive a measure oftension t as follows:

t=N((V ₁ /x ₂)−(V ₂ /x ₁))/(V ₂ r ₁ +V ₁ r ₂)   (3)

[0135] Thus for any given step rate for the motors, the appropriatecalibration factors x₁, x₂ can be looked up and used to derive a measureof the ribbon tension t. If the derived value of t is too high (above apredetermined limit), then a small step adjustment can be made to eitheror both of the motors to add a short section of ribbon to the length ofribbon between the spools. If the derived value of t is too low (below adifferent predetermined limit), then a short section of ribbon can beremoved from the length of ribbon between the spools. The controlalgorithms used to determine the correction amounts of ribbon added toor removed from the length of ribbon between the spools may be ofconventional form, for example the algorithms known as proportionalintegral derivative control algorithms MID control). The algorithms makeit possible to compare the measured tension t with predetermined upperand lower limits (the so-called deadband) and, if the measured tensionis outside these limits, the difference between the measured tension tand a “nominal demand” tension which is set at a level between the upperand lower limits may be calculated, the result of that calculation beingregarded as an error “signal”. This error “signal” is thenmathematically processed through the PED algorithms, which include aproportional gain constant, as well as integral and derivative factors.The mathematical processing results in a “correction” amount of ribbonthat needs to be added to or removed from the ribbon path between thespools during the next ribbon feed. This addition or removal of ribbonmaintains ribbon tension within acceptable limits.

[0136] In greater detail, the correction value may be calculated bycalculating the error (the difference between the nominal tension andthe measured tension) and dividing the error by a gain factor whichdepends upon the ribbon width. The greater the gain factor the tighterthe system will be as the nominal tension will be increased. The gainfactor is also dependent upon the ribbon width as the gain constants arechanged to take account of different ribbon widths. This is because atension which might cause considerable stretch in a narrow ribbon wouldcause minimal stretch in a wide ribbon and therefore the effects ofadding or removing ribbon from the length of ribbon between the spoolsis radically affected by ribbon stiffness. Successive cycles may adjustthe gain factor from a value nominally of 100 (tight) to a value ofnominally 80 (slack). For every consecutive tight or slack reading aftera first reading, an extra 0.1 mm correction can be added. An erroraccumulator is also maintained, and if the accumulated corrections(which are negative for tight and positive for slack) exceed plus orminus 2 mm then an additional 0.1 mm is added to the correction. Theseare the two integral components which enable the system to operate in astable manner and maintain ribbon tension at or close to the nominaltension.

[0137] The motor feed system splits the correction evenly between bothmotors in order to avoid large gaps between prints or over-printing onthe ribbon. The system does this by calculating the number of steps thathalf the correction amounts to for the stepper motor with the largestreal diameter. These steps are then re-calculated as a distance (relyingupon the known spool diameters) and subtracted from the originalcorrection amount. The resultant value is then used to calculate thecorrection for the motor driving the smaller diameter spool. Because themotor driving the smaller diameter spool has the smallest step size(when each step is converted to ribbon length) it can most accuratelyfeed the remaining distance. Thus the mechanism adjusts the tension byan amount that is as near as possible to that demanded by the originalcorrection.

[0138] It will be appreciated that if a particularly low tension readingis calculated by the above method, this can be taken by the controlsystem as indicating a fault condition, for example ribbon breakage, orthe ribbon becoming so slack that the system is most unlikely to be ableto effect adequate control. In such circumstances, the control systemcan output a “broken ribbon” predetermined low limits, such that whenthe measured tension t falls below this limit, the control system canhalt the printing process and assert appropriate fault outputs andwarning messages. Thus the system can offer valuable “broken ribbon”detection without the need for additional sensing arrangements.

[0139]FIG. 19 illustrates a circuit for calculating the ratio of thediameters of the spools 94 and 95 in the circuit of FIG. 18. Thepositive supply rail 96 of the power supply 80 (FIG. 18) is arranged tosupply current to four windings 97, 98, 99 and 100. Current is drawnthrough the windings 97 to 100 by transistors 101 which are controlledby motor control and sequencing logic circuits 102. The step rate iscontrolled by an input on line 103 and drive is enabled or disabled byan input on line 104 (high value on line 104 enables, low valuedisables). As before, if motor 92 is pulling, the drive circuit 108 forthat motor is enabled and therefore the rotation angle for the spoolbeing driven (94) is known. The drive circuit for the motor being pulled(93) is disabled (line 104 low). Thus motor 93 acts as a generator and aback-emf is generated across each of the motor windings 97 to 100. Thecomponents enclosed in box 108 of FIG. 19 corresponds to one of themotor drive circuits 81, 82 of FIG. 18. The voltage developed across thewinding 100 is applied to a level translator circuit 105 the output ofwhich is applied to a zero crossing detector 106 fed with a voltagereference on its positive input. The output of the zero crossingdetector 106 is a series of pulses on line 107. Those pulses aredelivered to the micro processor 89 of FIG. 18. By counting these pulsesfrom motor 93 over a known rotation angle of the drive motor 92 thespool diameter ratio can be calculated.

[0140] The method of monitoring ribbon tension as described withreference to FIG. 18 relies upon sampling current supplied to the motordrives 81 and 83 by sampling voltages developed across series resistors83 and 85. Preferably current is detected only during periods in whichthe ribbon has been advanced at a constant speed. In intermittentprinting systems, current is monitored during the return stroke of theprint head after each printing operation. During print head return, theribbon is also displaced. Thus the ribbon must be accelerated up to aconstant speed, advanced at that constant speed for a period duringwhich the current is monitored, decelerated and then positioned so as tominimise ribbon wastage. Driving a ribbon in this manner duringintermittent printing operations is a relatively simply matter as allthat is necessary is to ensure that the necessary motion of the ribbonincorporates a period of constant speed displacement during whichcurrent can be monitored. In continuous printing apparatus the problemis different as the ribbon is moving at a rate related to the substratespeed. Ribbon speeds of less than 50 mm per second are difficult toutilise as there is a tendency for the ink to cool before it can besecurely adhered to the substrate, and a wide range of substrate speedsabove 50 mm per second must be catered for. Nevertheless, in order tosave ribbon an amount of ribbon will always be returned to the supplyspool between successive printing operations. It is necessary to ensurethat the ribbon is returned in a manner such that the ribbon travels inthe return direction for a sufficient period of time at a constantvelocity to enable an accurate measurement of motor currents. It may bethat to achieve this it is necessary for the ribbon to be“over-returned” so that before the next printing operation the ribbonhas to be advanced to compensate for this over return. For bothcontinuous and intermittent printing over-return may be used to ensurethat sufficient ribbon is transported to provide an accurate measurementduring the tension measuring part of each printing cycle.

[0141] Preferably the motor currents are sampled over a period of timecorresponding to for example the travel of the ribbon through a distanceof at least 10nun at a constant velocity. For example the current couldbe sampled at regular intervals with the interval between successivesamples corresponding to for example one quarter of a step of the motor.The samples are added together and the sum is divided by the number ofsamples taken. This gives an average current which is reasonablyrepresentative of the power being drawn by the associated stepper motor.

[0142] An analysis of the waveforms of current supplied to steppermotors in the described embodiment shows that, in addition to thecurrent fluctuations resulting from the pulse width modulated nature ofmotor control, there is a substantial amount of variation in thewaveforms which will mean that individual samples may not berepresentative of the power being drawn by the motors. A more accuraterepresentation of that power can be obtained if the monitored signalsare passed through a low pass filter (not shown) before being averaged.

[0143]FIG. 19 illustrates one approach to the monitoring of changingspool diameters during ribbon usage. Alternative approaches are possiblehowever and one such alternative approach is described with reference toFIG. 20.

[0144] Referring to FIG. 20, A, and A, are the areas of spool 7 and 11(see FIG. 1) respectively, d is the inner diameter of the spools and Drand D, are the outer diameters of the spools at any given time. Hence:

A _(r) +As=constant   (4)

A _(r)=(D _(r)/2)²−(d/2)²   (5)

A ^(s)=(D _(S) /2) ²−(d/2)²   (6)

[0145] Substituting from (5) and (6) into (4) gives:

D _(r) ² +D _(S) ²=constant=D _(rc) ² +D _(sc) ²   (7)

[0146] Where D_(rc) and D_(sc) are rewind and supply spool diametersrespectively at initial calibration time. Current diameter ratio R =D_(r)/D_(s) Therefore rearranging this D_(s) = D_(r)/R And also D_(r) =RD_(S)

[0147] Substituting in (7) gives: $\begin{matrix}{D_{r}^{2} = {{D_{r}^{2}/R^{2}} = {D_{rc}^{2} + D_{SC}^{2}}}} \\{= {R_{c}^{2} + D_{sc}^{2} + D_{sc}^{2}}} \\{= {D_{SC}^{2}\left( {R_{c}^{2} + 1} \right)}}\end{matrix}$

[0148] where R_(c) is the ratio of rewind to the supply reel diameter atinitial calibration.

[0149] Therefore D_(r) ²(R²+1)/R²=D_(sc) ²(R_(c) ²+1) and D_(r)²=[R²/(R²+1)][D_(sc) ²(R_(c) ²+1)]

[0150] So knowing the initial calibration spool diameters ratio (R_(c)),supply spool diameters ratio (R_(c)), supply spool diameter atcalibration (D_(sc)) and the current spool diameters ratio (R), thecurrent diameter of either or both spools D_(r) or D_(s) can be derived.

[0151] In some applications it may be possible only to present acassette carrying a substantially empty take-up spool and asubstantially full supply spool of known outside diameter. In suchcircumstances it would not be necessary to determine the initial spooldiameters. In general however it is much to be preferred to directlymeasure the spool diameters as it is likely that machine operators willat least on occasions use non-standard spool configurations (for exampleribbon which has been partially used on an earlier occasion).

[0152] As an alternative to the approach described above with referenceto FIG. 18 and equations 1 to 3, it is possible to derive anapproximation of ribbon tension by relying upon the difference betweenthe currents drawn by the two motors. This difference current is afunction of the magnitude of the tension in the ribbon between the twomotors and may be used as a control parameter such that for example,when the magnitude of the difference in current falls outside anacceptable tolerance band, the previously assumed ratio of the spooloutside diameters is adjusted, resulting in a small change in the speedat which the two motors are driven. This speed adjustment compensatesfor the updated spool diameter ratio value. The “optimum” value of thedifference current and its tolerance band will change as the spooldiameters change. The appropriate value for a particular set ofcircumstances may be found from experimentation and stored in an optimumdifference current profile table which can be looked up as necessary.

[0153] No reference has been made in the above description to ribbonwidth, that is the dimension perpendicular to the direction of ribbonadvance. It may be appropriate to provide a user with the option tomanually enter a ribbon width value so as to enable the system to adjustthe predetermined tolerance limits and PID control gain constantsreferred to above to take account of tape-width dependentcharacteristics of the apparatus, e.g. to select different target limitsfor the measured tension t (equation 3).

[0154] As discussed above, in transfer printers it is necessary toaccurately position the printhead relative to the platen which supportsthe substrate to be printed if good quality print is to be produced,particularly at high printing speeds. The described embodiment of theinvention avoids the need to make these mechanical adjustments tooptimise printhead angle by making use of the fact that the printhead ismounted on a displaceable carriage.

[0155]FIG. 21 shows the roller 30, the printhead edge 32 and the peeloff roller 33 as shown in FIG. 5. The line 109 represents the adjacentedge of the cover plate 21. The broken line 110 represents the positionof a tangent to the roller 30 at the point of closest approach of theprinthead edge 32 (it will be appreciated that during printing asubstrate and a print ribbon will be interposed between the edge 32 andthe roller 30). The line 111 represents a radius extending from therotation axis 112 of the roller 30. The line 113 represents a notionalline through the axis 112 parallel to the edge 109. The line 113represents no more than a datum direction through the axis 112 fromwhich the angular position of the radius 111 corresponding to angle 114can be measured.

[0156] Angle 115 is the angle of inclination of the printhead relativeto the tangent line 110. This angle is critical to the quality of printproduced and will typically be specified by the manufacturer as havingto be within 1 or 2 degrees of a nominal value such as 30 degrees.Different printheads exhibit different characteristics however and it isdesirable to be able to make fine adjustments of say a degree or two ofthe angle 115.

[0157] It will be appreciated that the angle 115 is dependent firstlyupon the positioning of the printhead on its support structure andsecondly by the position of the tangent line 110. If the printhead wasto be moved to the right in FIG. 21, the angular position of theprinthead relative to the rotation axis of the roller will change. Thatangular position is represented by the magnitude of the angle 114. Asangle 114 increases, angle 115 decreases. Similarly, if the printheadshown in FIG. 21 was to be moved to the left, the angle 114 representingthe angular position of the printhead relative to the rotation axis ofthe roller would decrease and the angle 115 would increase. Thisrelationship makes it possible for an installer to make adjustments tothe printhead angle simply by adjusting the position adopted by thecarriage 36 on the track 22 (see FIG. 2) during printing. Thus aninstaller would initially position the printhead so that it would assumea nominal position in which the angle 114 would be approximately 90degrees. A test print run would then be used to assess print quality,the printhead would be displaced relative to the track, a fresh printrun would be conducted, and so on until the resultant print quality wasoptimised. There is no requirement for the installer to make mechanicaladjustments to the position of the printhead on its support.

[0158] The printing methods described with reference to FIGS. 13 to 16make it possible to increase printing speed by reducing the distancethat the printer ribbon has to be advanced between successive printingphases in successive printing cycles. FIG. 22 illustrates the appearanceof a printed substrate at the left hand side, and the appearance of anassociated printer ribbon after first, second, third and fourth printingoperations respectively. It will be seen that alternate images are madeup of slightly offset printed lines, that offset making it possible forthe printer head to traverse the printer ribbon as described withreference to FIGS. 13 and 16 such that successive images are generatedin part from overlapping portions of the printer ribbon. The speed ofadvance of the printer ribbon for a given substrate speed and imagereproduction rate can be doubled. In this context, the term “printingcycle” is used to refer to a fall cycle of activity which is performedin the interval between a printer head being first pressed into contactwith a printer ribbon so as to transfer ink from that ribbon to startthe formation of a first image until the printhead is again brought intocontact with the printer ribbon so as to initiate the transfer of inkwhich will form a second image. If the printing cycle relates to acontinuous printing machine, a full printing cycle includes an initialprinting phase in which the printhead is stationary and the printerribbon is transported with the substrate to be printed past theprinthead, and a subsequent non-printing phase during which thesubstrate continues to be transported past the printing head, theprinthead is retracted from contact with the print ribbon, the directionof transport of the print ribbon is reversed, and then the print ribbonis again fed forward until it is travelling in the direction of thesubstrate, whereafter the printing phase of the next printing cycle isinitiated. In an intermittent printer, the printing cycle is initiatedwith the substrate and ribbon stationary (unless the system is relyingupon slip printing), the printhead is advanced across the ribbon andsubstrate during a printing phase of the cycle, the printhead is thenretracted from the print tape and returned to its initial position, andthe substrate and printer ribbon are advanced in readiness for theinitiation of the next print cycle.

[0159] Thus, during the printing phase of each printing cycle, theprinthead traverses a predetermined length of ribbon either as a resultof displacement of the printhead relative to a stationary or slowermoving print ribbon, or as a result of displacement of the print ribbonrelative to the printhead. Thereafter the print ribbon is advanced apredetermined distance. The magnitude of that predetermined distance ofribbon advance is in many applications a limiting factor on the maximumspeed of the overall apparatus. In known printers the predetermineddistance of ribbon advance is generally at least as long as thepredetermined length of ribbon which is traversed by the printhead. Thedescribed apparatus makes it possible to operate in a manner in whichthe predetermined distance of ribbon advance is less than thepredetermined length of ribbon traversed by the printhead.

[0160] Referring to FIG. 22, the left had side of the Figure shows foursuccessive images deposited on a substrate, each image being the same.The right hand section of FIG. 22 shows the original image which has tobe reproduced on the substrate. The four intervening sections illustratethe appearance of the print ribbon after the printing of the four imagesshown on left hand side of FIG. 2. Assuming operation in intermittentprinting mode, the substrate is advanced by an equal distance betweeneach of the successive printing cycles. The substrate is stationaryduring each printing cycle, as is the ribbon. Each printing cycleincludes an initial printing phase during which the printhead is sweptacross the print ribbon so as to traverse a length of the ribboncorresponding to the length of the image formed on the substrate,followed by a further phase in which the printhead is returned to itsoriginal position and the ribbon is advanced a distance corresponding tohalf the length of the ribbon which is swept by the printhead during theprinting phase. During that first printing phase, only half of theprinting elements supported by the printhead are energised, and thus theimage deposited on the substrate is in the form of a series of parallellines. During the next printing phase, the printhead is again sweptacross the tape through a distance corresponding to the length of theimage, but during that motion printing elements of the printhead areenergised which contact different parts of the tape from those contactedby energised printing elements during the first printing cycle. At theend of the second printing cycle, the printhead is again returned to itsinitial position and the ribbon is advance by half the length of theimage formed on the substrate. Counting from the left in FIG. 22, thesecond, third, fourth and fifth sections of this Figure show theappearance of the print ribbon after each of the fist, second, third andfourth print cycles have been completed. It will be noted that all ofthe images formed on the substrate are substantially the same, the onlydifference between successive images on the substrate being that one ismade up of lines off set relative to lines forming the adjacent image.

[0161] The output represented in FIG. 22 is produced using a printheadin which the print element are arranged in a linear array with the oddnumbered printing elements in the array being allocated to one group andthe even numbered print elements in the array being allocated to theother group. This makes it possible to alternate between the groups sothat the distance advanced by the ribbon during each printing cycle isonly half of the length of ribbon from which ink is released during eachcycle. It will be appreciated that the printing elements could bearranged in three, four or more groups, the groups being energised in apredetermined cycle such that in for example the case of a three grouparrangement the distance advanced by the ribbon in each printing cyclecould be only one third of the length of printer ribbon swept by theprinthead in any one cycle.

[0162] Although this aspect of the present invention has been describedin detail in the context of intermittent printing, it will beappreciated that the same technique could be applied to a continuousprinting apparatus in which relative movement between the printingribbon and the printhead is the result of transport of the ribbon past astationary head rather than transport of a printhead relative to astationary ribbon.

1. A tape drive comprising two motors at least one of which is a steppermotor, two tape spool supports on which spools of tape may be mounted,each spool being drivable by a respective motor, and a controller forcontrolling the energisation of the motors such that the tape may betransported in at least one direction between spools mounted on thespool supports, wherein the controller is operative to energise bothmotors to drive the spools of tape in the direction of tape transport.2. A tape drive according to claim 1, wherein the controller is arrangedto control the motors to transport tape in both directions between thespools.
 3. A tape drive according to claim 1 or 2, wherein both of themotors are stepper motors.
 4. A tape drive according to claim 1, 2 or 3,wherein the controller is operative to monitor tension in a tape beingtransported between spools mounted on the spool supports and to controlthe motors to maintain the monitored tension between predeterminedlimits.
 5. A tape drive according to claim 4, wherein means are providedto monitor the power supplied to at least one of the motors and tocalculate an estimate of tape tension from the monitored power.
 6. Atape drive according to claim 5 as dependent upon claim 3, comprising apower supply, and a stepper motor drive means for supplying currentsequentially to windings of the stepper motors from the power supply,the power being monitored by means for monitoring the magnitude ofvoltage and/or current supplied to the motors and/or the motor drivemeans.
 7. A tape drive according to claim 6, comprising a regulatedpower supply providing a substantially constant voltage to the steppermotor drive means, the monitoring means monitoring the magnitude ofcurrent supplied to the stepper motor drive means.
 8. A tape driveaccording to claim 7, wherein each stepper motor is energised by arespective motor drive circuit, a respective low resistance resistor isconnected in series with each motor drive circuit, and voltage signalsdeveloped across the series resistors are monitored to monitor thecurrent supplied to the motors.
 9. A tape drive according to claim 8,wherein the voltage signals are converted to digital signals which aresupplied to a microcontroller which controls the generation of motorcontrol pulse trains which are applied to the motor drive circuits. 10.A tape drive according to claim 7, 8 or 9, wherein the means formonitoring current is operative to monitor current over a predeterminedperiod of time.
 11. A tape drive according to claim 10, wherein themonitoring means is operative only during periods in which tapetransport speed is substantially constant.
 12. A tape drive according toclaim 10 or 11, wherein the predetermined period of time corresponds toa predetermined length of tape transport.
 13. A tape drive according toany one of claims 5 to 12, wherein calibration data is recorded for theor each stepper motor, the calibration data representing powerconsumption for the stepper motor at each of a series of step ratesunder no tape load conditions, and a measure of tape tension iscalculated by reference to a measure of motor step rate, the calibrationdata related to the step rate, and power consumed by the motor.
 14. Atape drive according to any preceding claim, comprising means formonitoring the outside diameters of the tape spools, and means forcalculating tape tension by reference to the monitored diameters.
 15. Atape drive according to claim 14, wherein the outside diametermonitoring means is operative to monitor the outside diameter of thespools for each of a plurality of diameters which are mutually inclinedto each other.
 16. A tape drive according to claim 14 as dependant uponclaim 3 and claim 13, comprising means for calculating a measure oftension t, the controller being operative to maintain t betweenpredetermined upper and lower limits, where: t=N((V ₁ /x ₂)−(V ₂ /x₁))/(V ₂ r ₁ +V ₁ r ₂) and; V₁ is a measure of power supplied to a firstmotor acting as a take-up spool drive motor; V₂ is a measure of powersupplied to a second motor acting as a supply spool drive motor; r₁ isthe radius of a spool of tape driven by the first motor; r₂ is theradius of a spool of tape driven by the second motor; x₁ is acalibration factor for the first motor related to the step rate of themotor; x₂ is a calibration factor for the second motor related to thestep rate of the motor; and N is a calibration scaling factor.
 17. Atape drive according to claim 16, wherein the controller implements acontrol algorithm to calculate a length of tape to be added to orsubtracted from the tape extending between the spools in order tomaintain the value t between the predetermined limits and to control thestepper motors to add or subtract the calculated length of tape to thetape extending between the spools.
 18. A tape drive according to any oneof claims 7 to 15 as dependant upon claim 3, comprising means forderiving a measure of the difference or ratio between the currentssupplied to the two motors, and means for controlling stepping of themotors in dependence upon the difference or ratio measure.
 19. A tapedrive according to claim 18, wherein the controlling means maintainsmotor speed constant during periods in which the difference or ratiomeasure is within each of a series of tolerance bands defined betweenupper and lower limits, and means are provided for adjusting thetolerance bands in dependence upon the ratio of the outside diameters ofthe spools.
 20. A tape drive according to claim 18 or 19, wherein thecontrolling means implements a control algorithm to calculate a lengthof tape to be added to or subtracted from the tape extending between thespools in order to maintain the difference or ratio measure between theupper and lower limits and to control the stepper motors to add orsubtract the calculated length of tape to the tape extending between thespools.
 21. A tape drive according to claim 16, 17, 19 or 20, comprisingmeans for inputting a value corresponding to tape width, and means foradjusting the predetermined limits to take account of tape width.
 22. Atape drive according to claim 21 as dependent upon claim 17 or 20,wherein the control algorithm comprises gain constants, and the gainconstants are adjusted to take account of tape width.
 23. A tape driveaccording to claim 17, 20 or 22, wherein the control algorithm operatescyclically such that during one cycle the length of tape to be added orsubtracted is calculated and during a subsequent cycle the motors arecontrolled to adjust the amount of tape between the spools.
 24. A tapedrive according to claim 16, 17, 19, 20, 21, 22 or 23, comprising meansfor generating a fault-indicating output if the measure falls below aminimum acceptable limit below the lower limit. 25 A tape driveaccording to claim 14 or any claim dependant upon claim 14, wherein thediameter monitoring means comprises an optical sensing system includingat least one light emitter and at least one light detector arranged suchthat an optical path is established therebetween, a transport mechanismsupporting at least one part of the optical sensing system and drivableso as to cause the optical path to sweep across a space within whichspools to be measured will be located, and a controller operative tocontrol the transport mechanism, to detect positions of the transportmechanism in which the output of the detector changes to indicate atransition between two conditions in one of which the optical path isobstructed by a spool and in the other of which the optical path is notobstructed by that spool, and to calculate the spool diameters from thedetected positions of the transport mechanism in which the detectoroutput changes.
 26. A tape drive according to claim 25, wherein one ofthe emitter and detector is mounted on the transport mechanism, theother being fixed in position relative to the spools of tape.
 27. A tapedrive according to claim 25, wherein both the emitter and detector aremounted on the transport mechanism, the optical path between the emitterand detector being established by a mirror located at the side of thespools remote from the transport mechanism and arranged to reflect lightfrom the emitter back to the detector.
 28. A tape drive according toclaim 25, 26 or 27 as dependent upon claim 15, wherein the diametermonitoring means is operative to monitor the spool diameters with thespools in a first position, to rotate the spools to at least one furtherposition, and to monitor the spool diameters in the or each furtherposition, the calculated spool diameters enabling an accurate assessmentof spool eccentricity and outer circumference. 29 A tape drive accordingto claim 25, 26, 27 or 28, wherein the transport mechanism comprises aprint head transport mechanism of a transfer ribbon printer.
 30. A tapedrive according to claim 14 or any claim dependent upon claim 14,wherein the diameter measuring means comprises mean for calculating theratio of the diameters of the spools.
 31. A tape drive according toclaim 30 as dependant upon claim 3, wherein the ratio calculating meanscomprises means enabling a first stepper motor driving a take up spooland disabling a second stepper motor driving a supply spool such thatthe second stepper motor acts as a generator, means for generatingpulses from the second stepper motor, the pulse rate being proportionalto motor speed, means for detecting the generated pulses to produce ameasure of the rotation of the second stepper motor, means formonitoring stepping of the first stepper motor to produce a measure ofthe rotation of the first stepper motor, and means for comparing themeasures of the rotations of the motors to calculate the ratio of theoutside diameters of the spools.
 32. A tape drive according to claim 31,comprising means for calculating an updated diameter for at least onespool from a ratio between the spool diameters as initially monitored, acurrent ratio between the spool diameters, and the diameter of at leastone spool as initially monitored.
 33. A tape drive according to anypreceding claim incorporated in a transfer printer for transferring inkfrom a printer ribbon to a substrate which is transported along apredetermined path adjacent to the printer, the tape drive acting as aprinter ribbon drive mechanism for transporting ribbon between first andsecond ribbon spools, and the printer further comprising a printheadarranged to contact one side of the ribbon to press an opposite side ofthe ribbon into contact with a substrate on the predetermined path, aprinthead drive mechanism for transporting the printhead along a trackextending generally parallel to the predetermined substrate transportpath and for displacing the printhead into and out of contact with theribbon, and a controller controlling the printer ribbon and printheaddrive mechanisms, the controller being selectively programmable eitherto cause the ribbon to be transported relative to the predeterminedsubstrate transport path with the printhead stationery and displacedinto contact with the ribbon during printing, or to cause the printheadto be transported relative to the ribbon and the predetermined substratetransport path and to be displaced into contact with the ribbon duringprinting.
 34. A tape drive according to claim 33, wherein the drivemechanism is bi-directional such that ribbon may be transported from thefirst spool to the second spool and from the second spool to the first.35. A tape drive according to claim 34, wherein the printhead is mountedon a printhead carriage that is displaceable along the track, first andsecond carriages being provided which are interchangeable and are shapedsuch that with one carriage in position on-the track the printhead isdisposed so as to enable printing on a substrate travelling in onedirection along the substrate transport path and with the other carriagein position on the track the printhead is disposed so as to enableprinting on a substrate travelling in the other direction along thesubstrate transport path.
 36. A tape drive according to any precedingclaim incorporated in a printing apparatus comprising a housing, aprinthead mounted on a printhead support assembly which is displaceablerelative to the housing in a direction parallel to a print ribbon pathalong which a ribbon is driven by the tape drive, a first drivemechanism for displacing the printhead support relative to the housing,a roller which in use supports a substrate to be printed on the side ofthe ribbon path remote from the print head, a second drive mechanism fordisplacing the printhead relative to the printhead support assembly to aprinting position in which a portion of the printhead bears against theroller or any substrate or ribbon interposed between the printhead androller, and a controller for adjusting the first drive mechanism toadjust the angular position of the printhead relative to the rotationaxis of the roller.
 37. A tape drive according to any preceding claimincorporated in a printing apparatus comprising a housing, a printheadmounted on a printhead support assembly which is displaceable relativeto the housing in a direction parallel to a print ribbon path alongwhich a ribbon is driven by the tape drive, a first drive mechanism fordisplacing the printhead support relative to the housing, a peel offroller mounted on the printhead support assembly and displaceable withthe printhead in the said parallel direction, and a second drivemechanism for displacing the printhead relative to the printhead supportassembly and peel off roller between a ready to print position adjacentthe print ribbon path and a printing position in which the printheadwould contact a print ribbon on the path, wherein a cam mechanism isprovided which is engaged as a result of displacement of the printheadsupport assembly to a predetermined position and when engaged causesretraction of the printhead away from the ready to print position to aposition spaced from the peel roller and the print ribbon path.
 38. Atape drive according to claim 37, wherein the cam mechanism comprises aplate mounted on the housing and defining a slot, and a pin extendingfrom a pivotal member mounted on the printhead support assembly,engagement of the pin in the slot as a result of displacement of theprinthead support assembly to the predetermined position causing thepivotal member to rotate from a first position in which it supports theprinthead to a second position in which the printhead is free to move tothe position spaced from the peel roller and the print ribbon path. 39.A tape drive according to claim 38, wherein the pivotal member ismounted on a displaceable member mounted on the printhead supportassembly, displacement of the displaceable member from a retracted to anextended position when the pivotal member is in the first positioncausing the printhead to move from the ready to print from the printingposition.
 40. A tape drive according to any preceding claim incorporatedin a printing apparatus comprising a printhead, the tape drive servingas a printing ribbon drive mechanism for advancing a printing ribbonbetween the printhead and a path along which in use a substrate to beprinted is advanced, the print apparatus further comprising means forapplying the printhead to a ribbon supported in the drive mechanism, theprinthead comprising an array of printing elements each of which may beselectively energised to release ink from a portion of ribbon contactedby that element, and a controller for controlling energisation of theprinting elements and the advance of the ribbon so as to perform aseries of printing cycles each of which includes a printing phase duringwhich relative movement between the printhead and ribbon results in theprinthead traversing a predetermined length of ribbon and a non-printingphase during which the ribbon is advanced a predetermined distancerelative to the printhead, wherein the controller is arrangedselectively to energise different groups of printing elements duringsuccessive printing cycles, the groups of elements being distributed onthe printhead such that different groups contact different portions ofthe ribbon, and the controller is arranged to advance the ribbon suchthat the said predetermined distance of ribbon advance is less that thesaid predetermined length of ribbon, the groups of printing elementsbeing energised such that that ribbon is advanced by at least saidpredetermined length of ribbon in the interval between any two printingphases in which the same group of printing elements are energised.
 41. Atape drive according to claim 40, comprising two groups of printingelements, wherein said predetermined distance of ribbon advance is atleast half the said predetermined length of ribbon.
 42. A transferprinter for transferring ink from a printer ribbon to a substrate whichis transported along a predetermined path adjacent the printer,comprising a printer ribbon drive mechanism for transporting ribbonbetween first and second ribbon spools, a printhead arranged to contactone side of the ribbon to press an opposite side of the ribbon intocontact with a substrate on the predetermined path, a printhead drivemechanism for transporting the printhead along a track extendinggenerally parallel to the predetermined substrate transport path and fordisplacing the printhead into and out of contact with the ribbon, and acontroller being selectively programmable either to cause the ribbon tobe transported relative to the predetermined substrate transport pathwith the printhead stationery and displaced into contact with the ribbonduring printing, or to cause the printhead to be transported relative tothe ribbon and the predetermined substrate transport path and to bedisplaced into contact with the ribbon during printing.
 43. A transferprinter according to claim 42, wherein the drive mechanism isbi-directional such that the ribbon may be transported from the firstspool to the second spool and from the second spool to the first.
 44. Aprinter according to claim 42, wherein the ribbon drive mechanismcomprises at least one stepper motor to drive at least one ribbon spoolin the direction of tape transport.
 45. A printer according to claim 42or 43, wherein the ribbon drive mechanism comprises two stepper motorseach driving a respective one of the first and second ribbon spools inthe direction of tape transport.
 46. A printer according to claim 45,comprising means for monitoring ribbon tension and means for controllingthe stepper motors to maintain the monitored tension withinpredetermined limits.
 47. A printer according to claim 42, 43, 44, 45 or46, wherein the printhead drive mechanism comprises a stepper motorcoupled to the printhead.
 48. A printer according to claim 42, 43, 44,45, 46 or 47, wherein the printhead is mounted on a carriage that isdisplaceable along the track.
 49. A printer according to claim 48 asdependent upon claim 43, comprising first and second carriages which areinterchangeable and are shaped such that with one carriage in positionon the track the printhead is disposed so as to enable printing on asubstrate travelling in one direction along the substrate transport pathand with the other carriage in position on the track the print head isdisposed so as to enable printing on a substrate travelling in the otherdirection along the substrate transport path.
 50. A printer according toclaim 49, wherein a peel off roller is mounted adjacent the print head,the position of the peel off roller relative to the printhead beingreversible.
 51. An apparatus for measuring the diameters of two spoolsof tape mounted on a tape drive mechanism which is drivable to,transport tape between the spools, comprising an optical sensing systemincluding; at least one light emitter and at least one light detectorarranged such that an optical path is established therebetween, atransport mechanism supporting at least part of the optical sensingsystem and drivable so as to cause the optical path to sweep across aspace within which spools to be measured will be located, and acontroller operative to control the transport mechanism, to detectpositions of the transport mechanism in which the output of the detectorchanges to indicate a transition between two conditions in one of whichthe optical path is obstructed by a spool and in the other of which theoptical path is not obstructed by that spool, and to calculate the spooldiameters from the detected positions of the transport mechanism inwhich the detector output changes.
 52. A printing apparatus comprising ahousing, a printhead mounted on a printhead support assembly which isdisplaceable relative to the housing in a direction parallel to a printribbon path, a first drive mechanism for displacing the printheadsupport relative to the housing, a roller which in use supports asubstrate to be printed on the side of the ribbon path remote from theprinthead, a second drive mechanism for displacing the printheadrelative to the printhead support assembly to a printing position inwhich a portion of the printhead bears against the roller or anysubstrate or ribbon interposed between the printhead and roller, and acontroller for adjusting the first drive mechanism to adjust the angularposition of the printhead relative to the rotation axis of the roller.53. An apparatus according to claim 52, wherein the portion of theprinthead that bears against the roller or any substrate or ribboninterposed between the printhead and roller, is the portion of theprinthead that contains selectively energisable printing elements. 54.An apparatus according to claim 53, wherein the elements are linearlyarranged along the portion of the printhead.
 55. An apparatus accordingto 54, wherein the linear array of elements is arranged along an edge,or parallel in close proximity to an edge of the printhead.
 56. Aprinting apparatus comprising a housing, a printhead mounted on aprinthead support assembly which is displaceable relative to the housingin a direction parallel to a print ribbon path, a first drive mechanismfor displacing the printhead support relative to the housing, a peel offroller mounted on the printhead support assembly and displaceable withthe printhead in the said parallel direction, and a second drivemechanism for displacing the printhead relative to the printhead supportassembly and peel off roller between a ready to print position adjacentthe print ribbon path and a printing position in which the printheadwould contact a print ribbon on the path, wherein a cam mechanism isprovided which is engaged as a result of displacement of the printheadsupport assembly to a predetermined position and when engaged causesretraction of the printhead away from the ready to print position to aposition spaced from the peel roller and the print ribbon path.
 57. Aprinting apparatus comprising a printhead, a printing ribbon drivemechanism for advancing a printing ribbon between the printhead and apath along which in use a substrate to be printed is advanced, means forapplying the printhead to a ribbon supported in the drive mechanism, theprinthead comprising an array of printing elements each of which may beselectively energised to release ink from a portion of ribbon contactedby that element, and a controller for controlling energisation of theprinting elements and the advance of the ribbon so as to perform aseries of printing cycles each of which includes a printing phase duringwhich relative movement between the printhead and ribbon results in theprinthead traversing a predetermined length of ribbon and a non-printingphase during which the ribbon is advanced a predetermined distancerelative to the printhead, wherein the controller is arrangedselectively to energise different groups of printing elements duringsuccessive printing cycles, the groups of elements being distributed onthe printhead such that different groups contact different portions ofthe ribbon, and the controller is arranged to advance the ribbon suchthat the said predetermined distance of ribbon advance is less than thesaid predetermined length of ribbon, the groups of printing elementsbeing energised such that the ribbon is advanced by at least saidpredetermined length of ribbon in the interval between any two printingphases in which the same group of printing elements are energised.
 58. Amethod for controlling a tape drive comprising two motors at least oneof which is a stepper motor, two tape spool supports on which spools oftape may be mounted, each spool support being drivable by a respectivemotor, and a controller for controlling the energisation of the motorssuch that tape may be transported in at least one direction betweenspools mounted on the spool supports, wherein the controller energisesboth motors to drive the spools of tape in the direction of tapetransport.
 59. A method of controlling a transfer printer fortransferring ink from a printer ribbon to a substrate which istransported along a predetermined path adjacent the printer, wherein aprinter ribbon drive mechanism transports ribbon between first andsecond ribbon spools, a printhead is arranged to contact one side of theribbon to press an opposite side of the ribbon into contact with asubstrate on the predetermined path, a printhead drive mechanismtransports the printhead along a track extending generally parallel tothe predetermined substrate transport path and displaces the printheadinto and out of contact with the ribbon, and a controller controls theprinter ribbon and printhead drive mechanisms, the controller beingselectively programmed either to cause the ribbon to be transportedrelative to the predetermined substrate transport path with theprinthead stationary and displaced into contact with the ribbon duringprinting, or to cause the printhead to be transported relative to theribbon and the predetermined substrate transport path and to bedisplaced into contact with the ribbon during printing.
 60. A method formeasuring the diameters of two spools of tape mounted on a tape drivemechanism which is drivable to transport tape between the spools,wherein an optical sensing system including at least one light emitterand at least one light detector is arranged such that an optical path isestablished therebetween, a transport mechanism supporting at least partof the optical sensing system and is driven so as to cause the opticalpath to sweep across a space within which spools to be measured arelocated, and a controller which controls the transport mechanism detectspositions of the transport mechanism in which the output of the detectorchanges to indicate a transition between two conditions in one of whichthe optical path is obstructed by a spool and in the other of which theoptical path is not obstructed by that spool, the spool diameters beingcalculated from the detected positions of the transport mechanism inwhich the detector output changes.
 61. A method for adjusting theposition of a printhead in a printing apparatus comprising a housing, aprinthead support assembly on which the printhead is mounted and whichis displaceable relative to the housing in a direction parallel to aprint ribbon path, a first drive mechanism for displacing the printheadsupport relative to the housing, a roller which in use supports asubstrate to be printed on the side of the ribbon path remote from theprinthead, and a second drive mechanism for displacing the printheadrelative to the printhead support assembly to a printing position inwhich a portion of the printhead bears against the roller or anysubstrate or ribbon interposed between the printhead and roller, and acontroller for adjusting the first drive mechanism to adjust the angularposition of the printhead relative to the rotation axis of the roller,wherein the printhead is positioned adjacent the roller, a test print isperformed, the first drive mechanism is adjusted to change the angularposition of the printhead relative to the rotation axis of the roller, afurther test is performed, the adjustment and test processing beingrepeated until a satisfying print quality is achieved.
 62. A method forcontrolling the retraction of a printhead during printer ribbonreplacement in a printing apparatus comprising a housing, a printheadsupport assembly on which the printhead is mounted and which isdisplaceable relative to the housing in a direction parallel to a printribbon path, a first drive mechanism for displacing the printheadsupport relative to the housing, a peel off roller mounted on theprinthead support assembly and displaceable with the printhead in thesaid parallel direction, and a second drive mechanism for displacing theprinthead relative to the printhead support assembly and peel off rollerbetween a ready to print position adjacent the print ribbon path and aprinting position in which the printhead would contact a print ribbon onthe path, wherein when printer ribbon replacement is required theprinthead support assembly is displaced to a predetermined position, acam mechanism is positioned so as to be engaged as a result ofdisplacement of the printhead support assembly to the predeterminedposition, the cam mechanism when engaged causing retraction of theprinthead away from the ready to print position to a position spacedfrom the peel roller and the print ribbon path.
 63. A method forcontrolling a printing process in a printing apparatus comprising aprinthead, a printing ribbon drive mechanism for advancing a printingribbon between the printhead and a path along which in use a substrateto be printed is advanced, and means for applying the printhead to aribbon supported in the drive mechanism, the printhead comprising anarray of printing elements each of which may be selectively energised torelease ink from a portion of ribbon contacted by that element, whereinthe energisation of the printing elements and the advance of the ribbonis controlled so as to perform a series of printing cycles each of whichincludes a printing phase during which relative movement between theprinthead and ribbon results in the printhead traversing a predeterminedlength of ribbon and a non-printing phase during which the ribbon isadvanced a predetermined distance relative to the printhead, differentgroups of printing elements are selectively energised during successiveprinting cycles, the groups of elements being distributed on theprinthead such that different groups contact different portions of theribbon, and the ribbon is advanced such that the said predetermineddistance of ribbon advance is less than the said predetermined length ofribbon, the groups of printing elements being energised such that theribbon is advanced by at least said predetermined length of ribbon inthe interval between any two printing phases in which the same group ofprinting elements are energised.